197 
Unio ater, Lea. Trans. Am. Phil. Soc., vol. 3^ 'pl. 7, New iSeries. 
Ohs. I received several specimens of this shell from Mr. Bara- 
hino, who obtained them from Bayou Teche, Parish of St. Mary, 
Louisiana. It seem to be intermediate^ and almost connected by 
means of varieties^ to U. cariosus, nob., and ventricosus, Barnes ; 
like those species it has the posterior side prominent and very 
short, the anterior margin often truncate and the primary teeth 
often double in both valves ; but, however close this alliance may 
be, it can be distinguished by its somewhat different habit. It is 
differently colored, less cylindrical than cariosus, and proportion- 
ally longer and more cylindrical than ventricosus. 
In the young state the ligament margin appears to be coalite, 
and that of the older shell retains this union on its anterior portion . 
The name ater is preoccupied by Nilsson for a very distinct 
species. PL 43. 
Crepidula. — Shell free, univalve, irregular, ovate or oblong, 
convex, somewhat patelliform, with an epidermis } spire very short 
or obsolete, apex obvious, obliquely inclined, towards the margin : 
within horizontally divided about half the length by a diaphragm ; 
muscular impression lunate. 
Animal more or less depressed, oval, curved in the posterior 
part; mantle very thin, without marginal tentacula; foot not 
thick, small and rounded ; body each side of the head auriculated ; 
head emarginate before ; tentacula two, subcylindric, or subconic, 
slightly contractile, with the eyes near their exterior base ; mout h 
in the emargination, without teeth ; branchial cavity very large, 
situated on the anterior part of the back, containing a fascicle of 
long branchial filaments, which extend forward, and at tip float on 
the right side of the neck ; anus on the right side. 
Ohs. These are marine shells attaching themselves to various 
objects. At least many of the species have no locomotion, passing 
their whole life in one spot, and being moulded upon the surface 
of their resting place, exhibit its curvature and inequalities. 
Lamarck separated the species from the Linnean Patellse, form- 
ing a very natural and very distinct group. For although it ex- 
hibits a slight similarity to Septaria, Feruss., yet it is readily 
distinguished from that shell, which has not a proper diaphragm, 
the labium being only a little prominent and acute ; it is also a 
fresh water, operculated shell, allied to Neritina. 
