688 UNIO 
two heavy, curved, granular laterals; right valve with one, 
which is sometimes almost double ; beak cavities often impress- 
ed; muscle scars rather deep; nacre lurid purple. 
Length of type, a young shell, 78, height 50, diam. 38 mm. 
Length 103, height 65, diam. 47 mm. 
Georgia. 
Type locality, Satilla River, Wayne Co., Ga. 
Unto spissus Vea, Pr. Ac. Nat. Sei. Phila lll, 150, p, 112 
jlu Ae. Nat) Scr. Phila, AV, 21850; p- 208) ipl xxv hese. 
Obs., VII, 1859, p. 26, pl. xxv, fig. 89.—Sowerrsy, Conch. 
Icon., XVI, 1868, pl. LXxxvin, fig. 476.—Simpson, Syn., 
(900, p:"734- 
Margaron (Umio) spissus Lea, Syn., 1870, p. 41. 
The epidermis of this species, if it is a species, is darker than 
in any of the allied forms. The shell is very solid, the pseudo- 
cardinals quite small and for a shell of its solidity remarkably 
compressed; in old specimens they are solidly cross-striate. 
The posterior ridge is higher and more angulated than in any 
of the others. Yet I have seen shells that I cannot say with 
certainity whether they are this or U/. downiet. 
| Group otf Unio fisherianus. 
Shell elongated, rather thin, subcompressed, pointed behind 
and sometimes slightly biangulate; posterior ridge generally 
well developed; beaks low, the sculpture consisting of a few 
coarse, slightly irregular ridges, which run nearly parallel with 
the growth lines, generally heavier where they cross the pos- 
terior ridge; epidermis sometimes rayed, shining ; pseudocard- 
inals usually compressed; laterals long, straight and lamellar ; 
beak cavities very shallow and containing two or three dorsal 
sears at some distance behind the beaks; muscle scars well 
marked, the posterior ones elongated; nacre generally dull. 
Animal with the gills greatly elongated, inner the larger, 
more or less free from the abdominal sac; marsupium occupy- 
ing nearly or quite the whole length of the outer branchie ; 
palpi elongated; mantle thin, thicker on the edge. 
