Key:  Suborder Balanomorpha (Acorn barnacles)

Taken primarily from  Kozloff, 1987, 1996 p. 316 (Copyright 1987, 1996, University of Washington Press.  Used in this web page by permission of University of Washington Press)

Not in key: Whale barnacle Coronula diadema

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1a	Rostrum overlapped by the wall plates on both sides of it; cover (operculum) plates, when closed, fitting together in such a way that the lines between them intersect to form a cross; diameter, at base, not more than 1 cm (color generally gray or brownish gray; high intertidal)	Chthamalus dalli
1b	Rostrum overlapping the wall plates on both sides of it; cover plates, when closed, not fitting together in such a way that the lines between them intersect to form a cross; diameter, at base, often more than 1 cm, and in some species attaining 5 cm or more	2
2a	Tips of terga drawn out and curved in such a way that they form a beak	3
2b	Tips of terga not drawn out and curved in such a way that they form a beak	6
3a	Ridges on outside of wall narrow, closely spaced, and giving rise (especially in the lower half) to downward-pointing, fingerlike projections (this characteristic may be obscure on crowded or eroded specimens); base of shell not extensively calcified, and when the barnacle is knocked off the substratum, the base and some soft tissue generally remain on the substratum (common in mid- and low intertidal, and also in shallow subtidal; sometimes attaining a diameter of 4 cm or more)	Semibalanus cariosus
3b	Ridges of wall, if present, not giving rise to fingerlike projections; base of shell extensively calcified, and when the barnacle is knocked off a hard substratum, a calcareous scar remains on the substratum, but the rest of the barnacle is intact	4
4a	Diameter of shell commonly exceeding 5 cm, and sometimes exceeding 8 cm; exterior generally much eroded, so that the low ridges characteristic of the wall of young specimens become obscured (mostly subtidal, but also found on floating docks and in low intertidal on the open coast)	Balanus nubilus
4b	Diameter of shell rarely attaining 5 cm, and usually much smaller than this; exterior generally not much eroded, so that low ridges, when present, persist	5
5a	Tubes within wall plates with transverse septa, at least in the upper part of the wall (to see this feature, file the plates to expose the tubes, then blow out the powder); overlapping portions of plates not especially glossy; diameter rarely attaining 3 cm (subtidal)	Balanus balanus
5b	Tubes within wall plates lacking transverse septa; overlapping portions of plates usually glossy; diameter not often more than 3 cm, but sometimes reaching 5 cm (mostly subtidal, sometimes encountered on floating docks and on rocks in tidal streams)	Balanus rostratus
6a	Orifice small, generally only about 1/4 the diameter of the base; wall with ridges beginning at the edge of the orifice; subtidal, and almost always associated with hydrocorals of the genus Allopora	Solidobalanus engbergi
6b	Orifice from 1/3 to more than 1/2 the diameter of the base; when there are ridges on the wall, these do not necessarily begin at the edge of the orifice; includes intertidal and subtidal species	7
7a	Lines of contact between the terga and scuta approximately -shaped [line between left and right sides nearly straight, but line between terga and scuta sinuous, with an obvious loop) (photo)	8
7b	Lines of contact between the terga and scuta nearly straight or slightly sinuous, not -shaped	9
8a	Interior of base of shell with numerous centripetal ridges; interior surface of scutum with a prominent adductor ridge (this has a deep depression alonside it below the place where it meets the articular ridge); spur on tergum not as long as wide (mostly mid- and high intertidal; also on floating docks, where generally concentrated at or close to the water line; highly variable in general shape and in the extent to which the wall is ridged; crowded specimens tend to be tall and narrow and to lack ridges, whereas widely spaced specimens tend to be low and broad and to have prominent ridges)	Balanus glandula
8b	Interior of base of shell without centripetal ridges; interior surface of scutum without an obvious adductor ridge; spur on the tergum at least as long as wide (intertidal, with a wide vertical range, on rocks, wood, and sometimes on seaweed; not known to occur south of Washington)	Semibalanus balanoides
9a	Wall plates with internal tubes (these are exposed by filing the wall; they have transversesepta, at least in the upper part of the wall)	10
9b	Wall plates without internal tubes (subtidal)	Solidobalanus hesperius
10a	Exterior surface of scutum generally somewhat concave; wall with or without ridges (ridges are usually prominent only in young specimens); interior surface of scutum without an adductor ridge; spur of tergum wider than long (mostly subtidal; often found on ship bottoms and on floating docks; also low intertidal, and sometimes mid-intertidal, especially on relatively smooth boulders and cobble; not in brackish water	Balanus crenatus
10b	Exterior surface of scutum generally flat; wall usually without ridges; interior surface of scutum with an adductor ridge; spur of tergum longer than wide (restricted to brackish water; uncommon north of Oregon)	Balanus improvisus

  Balanus crenatus
  Balanus glandula
  Balanus nubilus
  Balanus rostratus
  Coronula diadema
  Semibalanus cariosus