Key to extant freshwater mussels of the Atlantic slope of North Carolina

How to use this dichotomous key...
"A dichotomous key consists of numbered, paired, statements of contrasting characteristics. In using a key, one reads the first couplet and decides which statement best describes the specimen at hand. Each statement indicates the number of the next couplet to be read. Couplet by couplet, the number of possible choices is reduced until the search terminates with the name of the taxon. If the specimen does not exactly match either statement, try both possible routes to an identification. It usually becomes evident that a wrong choice was made when the specimen clearly lacks the features mentioned in both statements of subsequent couplets." (Merritt and Cummins 1984).

Please Note: Red text is defined in the Glossary

a. shell more than 2.2 times as long as tall, valves have teeth . . . . . . . .2
b. shell usually less than 2.2 times as long as tall; in some species, valves may not have teeth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

a. surface of shell waxy and bright yellow when clean; very rarely has rays, if rays present then restricted to a few on the posterior slope; posterior ridge well rounded; rarely more than 60 mm long . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Elliptio lanceolata yellow lance
b. surface of shell usually dark yellow to brown-black, or green; rays may be numerous. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

a. concave ventral margin . . . . . . . . . . . . . . . . . . . . . . Elliptio folliculata pod lance
b. straight to slightly convex ventral margin . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

a. papillae along posterio-ventral mantle margin, sexually dimorphic shells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ligumia nasuta Eastern pondmussel
b. smooth posterio-ventral mantle margin, no sexually dimorphic shells . . . . . . .

lanceolate elliptio complex-
Elliptio viridula green lance
Elliptio fisheriana Northern lance
Elliptio angustata Carolina lance
Elliptio producta Atlantic spike
Elliptio emmonsii Roanoke spike

a. left valve has an interdental projection. . . . . . . . . . . . . . . . . . . . . . . . . 6
b. left valve lacking interdental projection . . . . . . . . . . . . . . . . . . . . . . . . .8

a. right valve with 2 lateral teeth. . . . . . . Alasmidonta heterodon dwarf wedgemussel
b. right valve with 1 lateral tooth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

a. usually less than 60 mm long. . . . . . . . . . . . Lasmigona subviridis green floater
b. often more than 60 mm long. . . . . . . . . .Lasmigona decorata Carolina heelsplitter

a. left valve without lateral teeth or lateral tooth vestigial. . . . . . . . . . . . . . . 9
b. left valve with 2 lateral teeth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

a. pseudocardinal tooth in left valve prominent or vestigial. . . . . . . . . . . . . 10
b. no evidence of teeth in left valve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

a. pseudocardinal tooth in left valve prominent. . . Alasmidonta undulata triangle floater
b. pseudocardinal tooth in left valve small to vestigial . . . . . . . . . . . . . . . . . .11

a. corrugations on posterior slope . . . . . . . . . . . . Alasmidonta varicosa brook floater
b. posterior slope smooth . . . . . . . . . . . . . . . Strophitus undulatus creeper

a. shell very thin, fragile; umbo does not extend or only slightly extends above hinge line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . 13
b. shell thin, fragile; umbo clearly extends above the hinge line. . . . . . . . . . . . 14

a. ventral margin relatively straight . . . . . . . . .Utterbackia imbecilis paper pondshell
b. ventral margin broadly curved. . . . . . . . .Anodonta couperiana barrel floater

a. anterio-ventral portion of the shell distinctly thickened below the pallial line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . .. . . . . . . . . . .Anodonta implicata alewife floater
b. no thickening of the shell below the pallial line. . .Pyganodon cataracta eastern floater

a. Many incurrent papillae dendritic or biramous . . . . . . . . . . . . . . . . . . . . . . . 16
b. Incurrent papillae simple, rarely biramous . . . . . . . . . . . . . . . . . . . . . . . . . . 19

a. All four gills marsupial in females, specimens rarely exceed 55 mm in length; young may have green rays; interdentum broad and plate-like in left valve; posterior ridge angular; periostracum with parchment-like texture. . . . . . . . . . Fusconaia masoni Atlantic pigtoe
b. Only outer gills marsupial in females; often > 100 mm in length . . . . . . . . . 17

a. Junction of dorsal and posterior margins strongly angled; periostracum not rayed; umbo very close to anterior end of shell; relatively small pseudocardinal teeth; tends to have a shaggy appearance . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Uniomerus sp. pondhorn
b. Junction of dorsal and posterior margins well rounded; individuals, particularly the young, often with distinct rays. . . . . . . . . . . . . . . . . . . .. . . . . .. . . . . . . . . . . . . . . . . . . . .18

18(17b)

a. Shell compressed, posterior slope unwrinkled, indistinct posterior ridge; shell usually with several centrally located plications; periostracum usually with reddish tint; shells tend to be smooth; pseudocardinal teeth large and chunky . .Elliptio roanokensis Roanoke slabshell

b. Shell relatively inflated, posterior slope with wrinkles, angular posterior ridge (this character is less prominent in northern extent of range), Atlantic Slope river basins other than Waccamaw River Basin . . . . . . . . . . . . . .Elliptio congaraea Carolina slabshell

a. Just anterior to posterior ridge on nacre side of shell, one to three small, thin ridges running from beak cavity to posterio-ventral margin of shell; just anterior to posterior ridge on outer shell surface, from 0 to 6 spines present on each valve; salmon colored nacre in anterior half of shell, iridescent bluish colored nacre in posterior half of shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . . .20
b. Shell without ridges on nacre, spines not present on shell . . . . . . . . . . . . . . 21

a. Posterior slope with wrinkles . . . . . . . . .Pleurobema collina James spinymussel
b. Posterior slope smooth . . . . . . . . . ..Elliptio steinstansana Tar River spinymussel

a. Species with significant sexual dimorphism (females with significantly expanded posterio-ventral margins of shells); females with papillae extending a significant distance along mantle margin anterior to incurrent aperture; marsupia of gravid females restricted to posterior half of outer demibranchs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

a. Females with a thick lump of papillation anterior to incurrent aperture; periostracum usually rough and black; has a biangulate posterior ridge; a small species rarely exceeding 30 mm in length . . . . . . . . . . . . . . .. . . .. . . ..Toxolasma pullus Savannah lilliput
b. Females with a distinct mantle flap anterior to incurrent aperture or mantle flap lacking but mantle often with dark pigmentation along papillate ventral margin; adults usually greater than 30 mm in length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23

a. Females with a distinct mantle flap anterior to incurrent aperture . . . . . . . . . 24
b. Females without a distinct mantle flap anterior to incurrent aperture . . . . . . .27

a. Shell glossy yellow; rays, when present, narrow and confined mostly to posterior slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lampsilis cariosa yellow lampmussel
b. Shell dark to golden yellow with broad greenish rays, especially toward the posterior end of the shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

a. Shell small, restricted to Lake Waccamaw . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . .Lampsilis fullerkati Waccamaw fatmucket
b. Adult shells usually greater than 55 mm in length . . . . . . . . . . . . . . . . . . . . 26

a. Nacre color salmon; mantle flap small and darkly colored . . . Lampsilis radiata two subspecies:
Lampsilis radiata radiata eastern lampmussel
Lampsilis radiata conspicua Carolina fatmucket

27(22b)

a. Marsupium of females with dark band on outer margin . . . . . . . . . . . . . . 28
b. Marsupium of females lacking dark band on outer margin . . . . . . . . . . . 29

a. Periostracum dark green to black (rarely yellow), occasionally with green rays; rarely over 30 mm long . . . . . . . . . . . . . . . . . . . . . . . . . Villosa constricta notched rainbow
b. Periostracum usually yellow with relatively wide, broken green rays; adults usually 50 mm or more in length . . . . . . . . . . . . . . . . . . . . . .Villosa delumbis eastern creekshell

a. Marsupium of females with ivory colored outer margin (lighter colored than rest of marsupium) . . . . . . . . . . . . . . . . . . . . . . . Villosa vaughaniana Carolina creekshell
b. Outer margin of female's marsupium same color as rest of marsupium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Leptodea ochracea tidewater mucket

a. Wrinkles on posterior slope; sharply angular posterior ridge, Waccamaw Basin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Elliptio waccamawensis Waccamaw spike
b. Posterior slope smooth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

31(30b)

a. ventral margin convex; posterior end broadly rounded and terminates half-way between dorsal and ventral margins . . . . . . . . Elliptio icterina complex variable spike complex
b. ventral margin straight or slightly convex; posterior end terminates closest to ventral margin (versus dorsal margin) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

a. Posterior ridge very swollen and angular; posterior slope narrow and clearly concave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Elliptio marsupiobesa Cape Fear spike
b. Width of shell highly variable; posterior slope usually wide and relatively flat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Elliptio complanata complex eastern elliptio complex