Key to Class Bivalvia

Taken primarily from  Kozloff, 1987, 1996  p 259 (Copyright 1987, 1996, University of Washington Press.  Used in this web page by permission of University of Washington Press)

Not in key:
  Nuttallia obscurata--dark mahogany clam
 

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Lower taxonomic level	Main Page	Alphabetic Index	Systematic Index	Glossary

1a	Shipworms--body wormlike, the shell greatly reduced and covering only a small part of the animal; burrowing in wood, forming tunnels lined by a whitish, calcareous deposit; siphonal tube with a pair of calcareous pallets (paddlelike or featherlike structures) for closing the opening of the burrow	Teredinidae
1b	Body not wormlike, the shell usually covering a substantial part of the animal; not burrowing in wood (exception: Xylophaga), although some species may nestle in holes made by other animals; siphonal tube without calcareous pallets	2
2a	Valves of shell decidedly unequal in form as well as size; 1 valve almost always permanently cemented to a hard substratum, such as rock, concrete, shell, or wood (some oysters, however, lie on a muddy substratum)	3
2b	Valves of shell essentially similar to one another, although 1 valve may be slightly smaller or less convex than the other; neither valve permanently cemented to the substratum (but the animal may be attached by a byssus of organic threads)	6
3a	Attached valve thin, flat or conforming to the shape of the substratum, with a prominent hole near the hinge (the byssal material that cements the animal to the substratum emerges through this hole)	Anomiidae: Pododesmus macroschisma (Pododesmus (Monia) cepio in key)
3b	Attached valve not thin and without a hole near the hinge	4
4a	Chamas:  Anterior and posterior  adductor muscles separate, their scars distinct on both valves	Chamidae
4b	Anterior and posterior  adductor muscles united into a single large muscle (photo of single adductor muscle scar in shell)	5
5a	Oysters--valves generally higher than long; without regular spaced ribs radiating from the umbones (if riblike extensions are present, they follow no particular pattern) (photo); valves without winglike extensions on both sides of the umbones; animal without eyes along the edge of the mantle	Ostreidae
5b	Rock scallops:  valves very thick, almost circular in outline; with rather regularly spaced, rough ribs radiating from the umbones; with winglike extensions on both sides of the umbones (these persist to a varying extent, and are not always prominent) (photo); with a row of eyes along the edge of the mantle (interior of both valves with a purple blotch near the hinge) (photo)	Pectinidae: Crassodoma gigantea (Hinnites giganteus)
6a	Scallops and limas--both valves with winglike extensions on both sides of the umbo (the extension on one side may be small, however)	7
6b	Neither valve with winglike extensions on both sides of the umbo	8
7a	Height about twice the length	Limidae
7b	Height not much, if at all, greater than the length	Pectinidae and Propeamussidae
8a	Anterior two-thirds of shell nearly globular and radially ribbed, the posterior third much narrower, almost cylindrical, comparatively smooth, and truncate; length rarely exceeding 1.5 cm (without definite ctenidia; strictly subtidal)	Cuspidariidae
8b	Shell not divided into a nearly globular anterior portion and a much narrower, almost cylindrical posterior portion; length frequently greater than 1.5 cm	9
9a	Hinge plate of both valves with taxodont dentition (photo)	10
9b	Hinge plate without taxodont dentition	11
10a	Hinge ligament  external; shell about as high as long; valves often with prominent radial ribs	Glycymerididae
10b	Hinge ligament internal or external; shell decidedly longer than high; valves without radial ribs (in Acila castrensis, however, there is radial sculpture that forms chevronlike patterns)	Order Nuculoida
11a	Exterior of both valves divided into 2 or 3 regions of distinctly different texture, the anterior region being rasplike or filelike for burrowing into hard clay, rock, shell, or wood (caution: Petricola pholadiformis, a member of the Petricolidae, burrows into clay, and its rasplike radial sculpture is especially prominent on the anterior half of the shell; this bivalve does not, however, have the other features of the family to which this choice leads); usually with a myophore; shell gaping anteriorly where the foot emerges, unless the gape is closed by a callus (typical of adults of some genera)	12
11b	Exterior of valves not usually divided into 2 or 3 regions of distinctly different texture (in some the valves are divided into 2 sectors by a groove that originates near the umbo, but the sectors have a similar texture), and generally without an anterior region which is rasplike or filelike for burrowing into hard clay, rock, shell, or wood (there are a few species that burrow into clay or rock, but with the exception of Petricola pholadiformis these do not have an anterior rasplike or filelike portion that is distinctly different from the rest of the shell); without a myophore;  shell not often gaping anteriorly, even though it may gape posteriorly	13
12a	Shell almost globular, but with a notch in the ventral side of the anterior portion of both valves; length less than 1 cm; boring in waterlogged wood (subtidal and rare; do not confuse with Bankia or Teredo, family Teredinidae)	Xylophagidae: Xylophaga washingtona
12b	Shell decidedly elongated, without a notch on the ventral side of the valves near the anterior end (the anterior end may, however, be abruptly truncate); burrowing in firm mud, clay, or shale	Pholadidae
13a	With a shiny brown periostracum that extends well beyond the edges of the valves (photo); shell so thin that it usually cracks when the periostracum dries (interior of valves with a slight radial rib just anterior to the scar of the posterior adductor muscle; dorsal and ventral margins almost perfectly parallel for much of their length; hinge without teeth; subtidal	Solemyidae
13b	Periostracum, whether shiny or not, not extending well beyond the edges of the valves; shell not usually so thin that it cracks when the periostracum dries (except in some species of Lyonsiidae)	14
14a	Shell as a whole very flat, the width not more than about one-sixth of the height; right valve flat, fitting into a slightly convex left valve; without a true hinge plate, the right valve having 2 or 3 ridges that articulate with grooves in the left valve	Pandoridae
14b	Shell as a whole at least one-fourth as wide as high; neither valve flat; with a true hinge plate, this sometimes provided with teeth or a chondrophore, or both	15
15a	Both valves with a single deep groove that runs from near the umbo to the posterior margin, this groove segregating a small posterodorsal sector; hinge without teeth, strictly subtidal	Thyrasiridae
15b	Neither valve with a deep groove that segregates a small posterodorsal sector; hinge with or without teeth; includes intertidal and subtidal species	16
16a	With a single adductor muscle scar not far from the center of each valve; height up to about 5 mm, slightly greater than the length (valves with prominent beaks, covered externally with a thick periostracum, pearly inside; attached by a byssus, usually to coralline algae)	Phylobryidae
16b	With 2 adductor muscle scars in each valve, although the anterior scar may be small and easily overlooked; height generally greater than 5 mm, and usually equaled or exceeded by the length	17
17a	Mussels--scars of the anterior and posterior adductor muscles decidedly unequal (the anterior scar is the smaller one, and may be scarcely noticeable (photo1photo2 ); umbones generally, but not always, much nearer the anterior end than the middle, hinge plate without teeth and without a chondrophore; usually attached to a hard substratum by a byssus of organic threads (includes some species that bore into rock or hard clay)	Mytilidae
17b	Scars of anterior and posterior adductor muscles of almost equal size (they may, however, be of idfferent shape); umbones not usually nearer to the anterior end than to the middle (but there are exceptions); hinge plates usually with teeth or with a chondrophore, sometimes both; usually not attached by a byssus of organic threads (but there are exceptions)	18
18a	Hinge plate of both valves without teeth and without a chondrophore; shell without radial ribs	19
18b	Hinge plate of 1 or both valves either with teeth or with a chondrophore, or both; shell with or without radial ribs	21
19a	Umbones so close together that they touch; the left one usually wearing a depression or hole in the right one; strictly subtidal	Thraciidae
19b	Umbones not so close together that they touch; intertidal and subtidal	20
20a	Interior of valves at least slightly pearly; with a continuous pallial line and pallial sinus (in Mytilimeria, which is typically embedded in compound ascidians, the pallial line and pallial sinus may not be distinct); tips of siphons not bright red	Lyonsiidae
20b	Interior of valves chalky, not at all pearly; pallial line discontinuous, pallial sinus absent; tips of siphons bright red	Hiatellidae (in part)
21a	Razor clams and jackknife clams--shell more than twice as long as high; periostracum glossy, usually yellowish-brown, yellowish olive, or greenish	22
21b	Shell not more than twice as long as high (except in Petricola pholadiformis); periostracum sometimes glossy and sometimes greenish, but not in species that are nearly or more than twice as long as high.	23
22a	About 4 times as long as high; dorsal margin slightly concave for its entire length	Solenidae: Solen (Solen) sicarius
22b	Less than 3 times as long as high; dorsal margin not concave	Cultellidae
23a	With radial ribs distributed over most of the shell (in Lucina tenuisculpta, Lucinidae, the radial ribs may not be evident except where the periostracum has been worn away; in Serripes groenlandicus, Cardiidae, and in Semele rubropicta, Scrobiculariidae, the ribs are often faint, and they may be apparent only on certain portions of the shell)	24
23b	Without radial ribs (except in Cryptomya californica, Myidae, which has faint radial ribs in the posterior portion, but which is distinctive in having a shelflike chondrophore projecting from the hinge plate of the left valve)	32
24a	Length of shell not appreciably greater than the height (not as much as 1 and one-sixth times the height), and sometimes slightly less than the height	25
24b	Length of shell definitely greater than the height, at least 1 and one-sixth times the height	28
25a	Posterior quarter of both valves with rather prominent concentric ridges as well as low radial ribs, so that the sculpture on this part of the valve is decidedly different from that over the rest of the surface, where only the closely spaced radial ribs are distinct	Cardiidae:  Nemocardium centifilosum
25b	Posterior quarter of valves not with concentric and radial sculpture that is decidedly different from the sculpture characteristic of the rest of the surface	26
26a	Radial ribs often faint, and may not be visible where covered by periostracum, but distinct on portions of the shell from which the periostracum has been worn away (shell chalky white, except where covered by a brownish or greenish periostracum; umbones near the middle; length up to about 1.5 cm; subtidal)	Lucinidae: Lucina (Parvilucina) tenuisculpta
26b	Radial ribs distinct even on portions of the shell where there is abundant periostracum	27
27a	Hinge with both cardinal teeth and lateral teeth; with more than 30 radial ribs	Cardiidae (in part)
27b	Hinge with only cardinal teeth; with fewer than 30 radial ribs	Carditidae (in part)
28a	Umbones  anterior to the end of the first 1/5 of the shell; anterior and ventral margins forming a nearly right angle (the corner of the angle, however, is rounded); with about 14 or 15 radial ribs; height up to about 1 cm	Carditidae: Glans carpenteri
28b	Umbones not as far forward as the end of the first 1/5 of the shell; anterior and ventral margins not forming nearly a right angle; with more than 15 radial ribs, even if some or all are faint; height usually more than 1 cm	29
29a	Shell about 2.5 times as long as wide; radial ribs especially promenent in the anterior half of the shell (burrowing in clay, firm mud, or soft shale)	Petricolidae: Petricola pholadiformis
29b	Shell nor more than twice as long as wide; radial ribs not obviously more prominent in the anterior half of the shell than elsewhere	30
30a	Without a pallial sinus (umbones near the second 1/5 of the shell; radial ribs low and broad, often obscure; periostracum tan or faintly greenish; generally somewhat glossy and tightly adherent; length up to 7 cm)	Cardiidae: Serripes grownlandicus
30b	With a pallial sinus	31
31a	Radial ribs low, often less prominent than the concentric growth lines; exterior of valves usually with pink radial rays; interior of valves sometimes with a purple stain at both ends of the hinge plate (length up to 3 cm; intertidal and subtidal)	Scrobiculariidae: Semele rubropicta
31b	Radial ribs almost always more prominent than the concentric growth lines; exterior of valves without pink radial rays; interior of valves without a purple stain at both ends of the hinge plate (but there may be a purple stain near the posterior end of each valve)	Veneridae (in part)
32a	With a shelflike or spoonlike chondrophore projecting from the hinge plate of the left valve (there is no chondrophore of this type on the right valve); hinge plate of both valves without teeth; hinge ligament largely or completely internal	Myidae
32b	Without a shelflike or spoonlike chondrophore projecting from the hinge plate of the left valve, but sometimes with a socketlike chondrophore in both valves (photo);hinge plate of both valves with teeth; hinge ligament generally external but sometimes almost completely internal	33
33a	Valves without a pallial sinus	34
33b	Valves with a pallial sinus (photo)	43
34a	Valves with pronounced concentric ridges, and not nearly circular in outline	Astartidae
34b	Valves with faint concentric growth lines, but without pronounced concentric ridges (except in Luconoma annulata, which is distinctive in having nearly circular valves)	35
35a	Height approximately equal to the length, the outline of the valves nearly circular	36
35b	Length decidedly greater than the height, the outline of the valves not nearly circular	38
36a
36b
37a
37b
38a	Ventral margin of valves nearly straight or slightly concave; up to about 2 cm long; attached by a byssus to the ventral surface of the mud shrimp Upogebia pugettensis or to the ventral surface of polychaetes of the genus Aphrodita	Montacutidae: Pseudopythina rugifera
38b	Ventral margin of valves convex; up to 2.5 cm long (Kellia suborbicularis), but generally less than 1 cm; sometimes nestling in holes made by other invertebrates, sometimes closely associated with other invertebrates, but not attached to Upogebia or Aphrodita	39
39a
39b
40a
40b
41a
41b
42a
42b
43a	Pallial line consisting of a series of discontinuous scars (with 1 small tooth on the hinge plate of each valve; shell gaping at both ends; posterior end truncate; length up to 7 cm	Hiatellidae: Panomya beringiana
43b	Pallial line continuous (photo)	44
44a	With a single tooth on the hinge plate of each valve; shell large, the length sometimes exceeding 15 cm, but not large enough to accomodate the immense siphon, and gaping widely except in the region of the hinge	Hiatellidae: Panopea abrupta
44b	With more than 1 tooth on the hinge plate of each valve; shell generally less than 10 cm long (except in Tresus), large enough to accomodate the siphon when it is withdrawn, gaping only at the posterior end, if at all	45
45a	Shell gaping broadly at the posterior end (the width of the gape is more than 1/4 the width of the shell) (photo); length commonly exceeding 12 cm	Mactridae: Tresus
45b	Shell closing tightly or gaping only slightly at the posterior end; length rarely exceeding 10 cm	46
46a	Hinge ligament almost completely internal	Mactridae (in part)
46b	Hinge ligament largely external	47
47a	With 3 cardinal teeth (and sometimes a pair of lateral teeth) in both valves	Veneridae (in part)
47b	With fewer than 3 cardinal teeth in at least 1 valve	48
48a	With 3 cardinal teeth in 1 valve, 2 in the other	49
48b	With 2 cardinal teeth in both valves	50
49a	Valves oval, rather delicate, polished; siphons not tipped with bright purple	Cooperellidae: Cooperella subdiaphana
49b	Valves somewhat elongated, and usually narrower posteriorly than anteriorly, rather thick, chalky (often distorted as a result of conforming to the shape of pholad burrows in which this species nestles); siphons tipped with bright purple	Petricolidae: Petricola (Rupellaria) carditoides
50a	Outline of valves almost elliptical (about twice as long as high); hinge plate of both valves, behind the umbo, with a thin dorsal extension to which the ligament is attached; exterior of valves usually with pinkish or purplish radial rays; siphons short, united; length up to about 7 cm [Note:  Another member of this family, Nuttallia obscurata, is not in this key but has recently been introduced to the area]	Psammobiidae: Gari (Gobraeus) californica
50b	Outline of valves not almost elliptical (the 2 ends are different, one being less evenly rounded than the other); hinge plate of valves without a thin dorsal extension; exterior of valves without pinkish or purplish radial rays (various tints, however, may be present internally or externally); siphons  long and separate; maximum length from 1 to 10 cm	Tellinidae