Dichotomous Key

A dichotomous key for identifying freshwater mussels of the New York Metro Area and New Jersey.

1	a	shell with a very sharp posterior ridge, without hinge teeth, shaped like a marine mussel, Mytilus, generally less than 30 mm, and attached to a hard substrate with byssal threads	Dreissena polymorpha
	b	animal without byssal threads attaching adult to substrate, with or without teeth, not shaped like a marine mussel	2
2	a	valves with cardinal teeth and two sets of lateral teeth- anterior or posterior	3
	b	valves with one set of lateral and pseuocardinal teeth, or without teeth	4
3	a	valves with serrated lateral teeth; adults greater than fifty mm. in length	Corbicula fluminea
	b	valves with smooth lateral teeth; generally less than twenty five mm. in length	Sphaeriidae *
4	a	hinge teeth absent or nearly absent	5
	b	hinge teeth present	8
5	a	beaks not projecting above the hinge line; shell extremely thin	Utterbeckia imbecillis
	b	beaks projecting above the hinge line	6
6	a	beak sculpture double looped; pseudocardinal teeth completely absent	7
	b	beak sculpture concentric and coarse, pseudocardinal teeth represented by a raised thickening near the beaks	Strophitus undulatus
7	a	nacre salmon or copper colored, shell prominently thickened along the anterior ventral margin below the pallial line	Anodonta implicata
	b	nacre bluish or white, shell uniformly thin, epidermis often greenish	Pyganodon cataracta
8	a	lateral teeth absent or reduced, neither functional nor interlocking	9
	b	lateral teeth well developed, functional and interlocking	11
9	a	fine transverse ridges on posterior slope present, posterior ridge rounded, pseudocardinal teeth reduced and elongate, shell rounded and usually less than 70mm long	Alasmidonta varicosa
	b	fine traverse ridges on posterior slope absent, pseudocardinal teeth strong and triangular	10
10	a	pseudocardinal teeth strong and triangular with rough surfaces, shell small to medium, less than 80mm, triangular to ovate	Alasmidonta undulata
	b	pseudocardinal teeth pronounced and smooth, shell large to 150mm, elongate, oval and kidney-shaped, shell very thick and heavy, periostracum blackish	Margaritifera margaritifera
11	a	right valve with two lateral teeth, shell less than 55mm	Alasmidonta heterodon †
	b	right valve with one lateral tooth	12
12	a	height/length less than or equal to 0.48, posterior ridge prominent, posterior end of shell subangular and drawn to a blunt point; pseudocardinal teeth elongate and delicate, shell usually less than 110 mm in length	Ligumia nasuta
	b	height/length greater than 0.48, posterior end not pointed	13
13	a	nacre may be purple, shell subrhomboid with well defined posterior ridge and slope, very common	Elliptio complanata
	b	nacre white or colored but not purple	14
14	a	left valve with small interdental tooth, giving the appearance of three pseudocardinal teeth, shell more or less compressed and subrhomboid in outline, dark green with numerous green rays or brown, adult shell less than 65 mm long, posterior ridge rounded	Lasmigona subviridis
	b	left valve without interdental tooth, valve appears to have only two pseudocardinal teeth	15
15	a	adult shell usually less than 80 mm in length, and thin, hardly thicker anteriorly than posteriorly, epidermis dull yellow without rays or with fine rays all over the shell, rays may not be well defined; hinge teeth delicate; in or near tidewaters	Leptodea ochracea
	b	adult shell often greater than 80 mm in length, much thicker anteriorly than posteriorly, may have obvious, well defined broad color rays; hinge teeth strong	16
16	a	shell without green rays or with green rays restricted to the posterior slope, periostracum yellow, clear and glossy	Lampsilis cariosa
	b	shell with rays all over (may be obscured in old adults), height/length less than 0.60 in males and in most females, posterior ridge low and rounded, beak not prominent; periostracum greenish and slightly roughened by wrinkles parallel to the growth lines	Lampsilis radiata

* Sphaeriidae, the fingernail or pill clams, are beyond the scope of this handbook.

† A. heterodon is protected as a Federal and State endangered species, and the killing of this species for any purpose, including identification, is strictly prohibited. In nearly all instances, destructive sampling of metro area freshwater mussels is not necessary for identification and is strongly discouraged; many other area species are protected by state regulation.