14 AMERICAN JOURNAL 



tudinal furrow ; cardinal plate profoundly dilated, rugoso-plicate ; 

 inner margin finely crenulated. 



Locality. — Virginia. J. A. Marks. 



Observations. — This is the thickest and most ponderous bivalve 

 of its size which has yet been found in America. Though its 

 length is only four and three-fourths, its diameter is four and one- 

 fourth inches. The hinge is correspondingly strong, the teeth 

 thick and the intervening pits very profound ; the nympha in the 

 right valve very prominent and profoundly tuberculato-rugose, 

 which fits into a cavity of the opposite valve behind the promi- 

 nent part of the nympha. 



Allied to M. tridacnoides, Lam., from which its hinge, thick- 

 ness, and much finer marginal crenulations readily distinguish it. 

 Its external radiating grooves are narrower and more angular 

 than those of the former species. 



CYTHERIOPSIS, Conrad. 



The figure of Grrateloupia irregularis in Homes' Foss. Mollusk. 

 des Wiener Beckens, pi. 16, fig. 5, shows a palleal impression of 

 the same character as that of (r. donaciformis, Desmoulins ; 

 joining the posterior cicatrix at its lower margin it runs towards 

 the ventral margin of the shell a short distance, and then sud- 

 denly curves upwards and forms a wide sinus, the anterior ex- 

 tremity of which is much beyond the middle of the valves. This 

 character forms a marked contrast to the form of the palleal line 

 in Cytheriopsis, which joins the anterior extremity of the cica- 

 trix, and has a short rounded or subangular sinus. 



SCROBICULARIID^. 



SEMELE, Schum. 



S. CAROLINENSIS, Conrad. 



Description. — Suborbicular, inequilateral ; length and height 

 nearly or quite equal. 



Ampliidesma orbiculata, Tuomey and Holmes, (not Say.) — 

 Pliocene Foss. of S. Carolina, pi. 23, fig. 4. 



Observations. — This shell has a thicker and broader cardinal 

 plate than the orbiculata, and may be distinguished from it by 

 its outline, the latter species being considerably longer in pro- 

 portion to the height. 



ABRA, Leach. 

 A. NUCULIFOEMIS, Conrad. 



Description. — Subtriangular ; very inequilateral ; anterior side 

 somewhat produced, acutely rounded at the extremity. 



