Key to Class Polyplacophora (Chitons)

Note:  The key below was taken primarily from  Kozloff, 1987, 1996 p. 185 (Copyright 1987, 1996, University of Washington Press.  Used in this web page by permission of University of Washington Press)
Aaron Baldwin has recently created an alternate key to polyplacophorans which I have found to be very useful, more up-to-date, and often easier to use than is the Kozloff Key.
The key has many excellent photos of the distinguishing characteristics of the different chiton species.
Aaron has kindly made his key available as a pdf file.  The Baldwin key to chitons can be accessed here.
 

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1a	All plates completely covered by the tough, reddish brown girdle; length commonly exceeding 20 cm	Cryptochiton stelleri
1b	All 8 plates visible; length not often exceeding 10 cm	2
2a	Plates 2-7 divided along the midline by a strip of cartilagelike tissue (length up to 2.5 cm; rare, and not likely to be found south of British Columbia)	Schizoplax brandtii
2b	None of the plates are divided along the midline	3
3a	Girdle uniformly black or brownish black, covering about two-thirds of the area of each plate (visible portions of plates 2-8 not much wider than long) (girdle smooth; common mid-intertidal species)	Katharina tunicata
3b	Girdle not uniformly black or brownish black (except in some specimens of Lepidochitona fernaldi, choice 10b), and not covering as much as two-thirds of the area of each plate	4
4a	Plate 8, measured along the midline, about twice as long as plate 1	5
4b	Plate 8 not nearly twice as long as plate 1	6
5a	Dorsal surface of girdle with obvious overlapping scales, these somewhat pointed and up to about 0.3 mm long; plate 1 with 10-14 slits; plates 2-7 with 1-3 slits on both sides; foot pale; under rocks in mid-intertidal (not likely to be found north of California)	Stenoplax heathiana
5b	Dorsal surface of girdle with very small scales (mostly less than 0.2 mm long), these not at all pointed; plate 1 with 8-10 slits; plates 2-7 with 1 slit on both sides; foot (in life) bright orange; mostly subtidal (not likely to be found north of Oregon)	Stenoplax fallax
6a	Dorsal surface of girdle smooth or granular (granules, visible at a magnification of about 10x, generally have a circular outline), but without overlapping scales (scales usually have an oval outline), hairs, or spicules (there may, however, be spicules along the margin of the girdle)	7
6b	Dorsal surface of girdle with overlapping scales (clearly visible as such at a magnification of 10x,)(photo), or with hairs or spicules (when hairs or spicules are present, the girdle may also be granular)	11
7a	Plates with prominent wavy or zigzag markings of white, yellow, pink, purple, or other bright colors; commonly more than 3 cm long	8
7b	Plates with various color patterns, but not predominantly with wavy or zigzag markings of bright colors; length not often exceeding 2 cm	9
8a	Anterior portions of plates 2-7 with wavy, light lines running transversely across the midline; girdle generally with subdued green blotches; mostly subtidal	Tonicella insignis
8b	Anterior portions of plates 2-7 without light lines running across the midline; girdle generally with conspicuous whitish, yellow, or orange blotches; common low intertidal and subtidal species (picture)	Tonicella lineata
9a	Gills with about 9 ctenidia on both sides, bordering the posterior third of the foot; plates without slits along their inserted margins (the anterior margin of at least 1 plate must be exposed if this feature is to be seen) (low intertidal and subtidal)	Leptochiton rugatus
9b	Gills usually with more than 9 ctenidia on both sides; bordering considerably more than the posterior third of the foot; plates with slits along their inserted margins	10
10a	Gills with 18-25 ctenidia on both sides, extending at least 4/5 of the length of the foot; girdle usually with alternating light and dark bland or with light spots; valves rarely eroded; not brooding young; eggs about 210 microns in diameter, surrounded by transparent, cone-shaped hulls; mostly low and mid-intertidal but occurring in tidepools at higher levels	Lepidochitona dentiens
10b	Gills with 8-17 ctenidia on both sides, extending along not more than 3/4 the length of the foot; girdle usually dark, sometimes with white spots, but without alternating color bands; valves often eroded; brooding young; eggs about 260-280 microns in diameter, and nearly smooth, high intertidal	Lepidochitona fernaldi
11a	Dorsal surface of girdle without overlapping scales, but with spicules or flexible or stiff hairs	12
11b	Dorsal surface of girdle with overlapping scales (photo), but without spicules or hairs (spicules may, however, be present along the margin of the girdle)	16
12a	Dorsal surface of girdle with crystalline spicules, but without hairs	13
12b	Dorsal surface of girdle with hairs	23
13a
13b
14a
14b
15a
15b
16a	Lateral areas of plates 2-7 conspicuously raised above the central area, swollen, and with pronounced tubercles; length of body almost 3 times the width (not likely to be found N of CA)	Callistochiton crassicostatus
16b	Lateral areas of plates 2-7 not conspicuously raised above the central area and without pronounced tubercles (but there may be small nodules in the lateral areas) (photo); length of body not more than twice the width	17
17a	Lateral areas of plates 2-7 with hemispherical or nearly globular nodules (photo); central area usually with longitudinal ridges (photo), or with longitudinal rows of pits separated by incipient ridges	19
17b	Lateral areas of plates 2-7 without nodules; central area without longitudinal ridges or rows of pits, although there may be some delicate pitting	18
18a
18b
19a	Central area of plates 2-7 with longitudinal rows of shallow but conspicuous pits, the longitudinal ridges between the pits often scarcely noticeable	Lepidozona retiporosa
19b	Central area of plates 2-7 with rather distinct longitudinal ridges (photo), even if rows of shallow pits are present between these	20
20a	Longitudinal ridges near the midline of plates 2-7 diverging anteriorly to the extent that they form a wedge-shaped area; ridges prominent in all or much of the central area of plates 2-7	21
20b	Longitudinal ridges near the midline of plates 2-7 not diverging anteriorly (or diverging only on plate 2); ridges in the central area of plates 2-7 indistinct (rare species)	22
21a	Color olive or gray, often darker in the central portions of the plates; plates 1 and 8 with 11 slits; open coast	Lepidozona cooperi
21b	Color variable, but generally reddish (sometimes greenish or purplish), occasionally speckled with cream, tan, or brown, and often with a lighter color in the central portions of the plates; plate 1 with 10 or 11 slits, plate 8 with 10 slits; inland waters as well as on the open coast	Lepidozona mertensii
22a
22b
23a	Girdle much wider in the anterior part of the body than elsewhere (picture)	Placiphorella velata Carpenter in Dall, 1879
23b	Girdle not appreciably wider in the anterior part of the body than elsewhere	24
24a	Dorsal surface of girdle obviously granular, as well as with hairs	25
24b	Dorsal surface of girdle not obviously granular	26
25a	Central areas of plates 2-7 with a few nearly parallel longitudinal ridges on both sides of the median ridge; branching of sutural hairs (those in line with the sutures between plates) not limited to 1 side of the hairs; length not often greater than 1 cm (rare)	Dendrochiton semiliratus
25b	Central areas of plates 2-7 without parallel longitudinal ridges; sutural hairs either not branched or branched only on 1 side; length up to about 3 cm	Dendrochiton flectens
26a	Hairs stout, rubbery, and stiff (common intertidal species; length up to about 7 cm)	Mopalia muscosa
26b	Hairs flexible, not thick	27
27a	Length of some hairs, in addition to the sutural hairs (those in line with the sutures between plates), exceeding the width of the girdle; body length not often more than 1.2 cm; rare	28
27b	Length of girdlehairs, except perhaps the sutural hairs, hot exceeding the width of the girdle and generally not equaling it (in the rare Mopalia inporcata, some of the hairs are as long as the girdle is wide; moreover, young individuals of various species, until they have reached a length of at least 1 cm, may have proportionately long hairs)	29
28a
28b
29a	Lateral areas of plates 2-7 separated from the central area by a slight, nearly smooth ridge, or by a series of tubercles that are not considerably larger than those that form the more or less longitudinal rows on the central area and the oblique rows on the lateral areas	30
29b	Lateral areas of plates 2-7 (or most of them) separated from the central area by a series of enlarged tubercles or by an obvious ridge of partly coalesced enlarged tubercles (the enlarged tubercles are located at the points where the more or less longitudinal rows of tubercles on the central area meet the oblique rows on the lateral areas)	32
30a	Plates 2-7 smooth, without rows of tubercles and without pits; girdle with alternating light and dark bands, sparsely covered with branching hairs that are so delicate that the girdle appears naked; plates 2-7 generally brick red, with zigzag turquoise markings in the lateral areas; plate 1 streaked with white; plate 8 with a triangular white patch extending posteriorly from its apex (common subtidal species)	Mopalia laevior
30b	Plates 2-7 with rows of fine tubercles or with pits (the tubercles or pits are clearly visible with low magnification); girdle of a uniform color, spotted, or banded, with hairs that are easily seen, even when sparse; plates without the color patterns described in choice 30a	31
31a	Plates 2-7 without obvious rows of raised tubercles, but with longitudinal rows of pits, which are especially pronounced in the central areas; lateral areas separated from the central areas by a slight, nearly smooth ridge; hairs on girdle abundant, robust and tubular; branches of hairs, if present, distributed along the entire length of the hairs; cleft in girdle at posterior end scarcely evident; interior surface of plates blue or white, chalky; ventral surface of girdle orange, even in preserved specimens (common intertidal species)	Mopalia lignosa
31b	Plates 2-7 with obvious rows of tubercles (clearly visible with low magnification); lateral areas separated from the central band by a series of tubercles that mark the places where the more or less longitudinal rows on the central areas meet the oblique rows on the lateral areas; hairs on girdle sparse, very slender; branches of hairs, if present, originating only from the proximal portions of the larger hairs; cleft in girdle at posterior end pronounced; interior surface of plates white, not chalky; ventral surface of girdle not orange (common intertidal species, especially in bays)	Mopalia hindsii
32a	Length of many hairs on the girdle equal to the width of the girdle; hairs abundant and scattered randomly, hairs with many branches, some of which are half the length of the hairs from which they originate; cleft in girdle at the posterior end not evident (rare)	Mopalia imporcata
32b	Length of hairs on the girdle less than the width of the girdle; hairs either scattered randomly or (in Mopalia porifera) appearing to be in a definite pattern; branches of hairs, when present, no more than one-fourth the length of the hairs from which they originate; cleft in girdle at posterior end may or may not be evident.	33
33a	Hairs on girdle arranged in 3 staggered rows, the hairs of 1 row being sutural (in line with the sutures between plates 2-7); without a definite cleft in the girdle at the posterior end (branches of hairs, often calcified, originating on 1 side of each hair; rare)	Mopalia porifera
33b	Hairs on girdle scattered randomly; with a definite cleft in the girdle at the posterior end	34
34a	Hairs on girdle sparse and so short (up to 0.5 mm long) that one may at first think that there are none (hairs with fine filaments coming mostly from the base, each filament tipped by a glassy spicule)	Mopalia swanii
34b	Hairs on girdle usually abundant and up to 3 or 4 mm long, unless broken off	35
35a	Hairs on girdle strap-shaped, up to 3 mm long, with prominent glassy spicules along 1 side (usually the medial side); color variable, and sometimes similar to that of the next species (comon intertidal and shallow subtidal species)	Mopalia ciliata
35b	Hairs on girdle not obviously strap-shaped, up to 4 mm long, with filamentous branches tipped by glassy spicules; color almost always including bright turquoise and orange or reddish brown markings, especially on plate 2	Mopalia spectabilis