140 AMERICAN JOURNAL 



lias a slight farrow or fold radiating from tlie beak ; anterior 

 extremity acute ; ventral margin rounded. 



This small species is very perfect, with both valves connected. 

 The figure is much enlarged. 



NUCULA, Lam. 



N. sPHENiopsis, nob. — pi. 10, fig. 13. 



Description. — Obliquely ovate-triangular, slightly ventricose ; 

 posterior side cuneiform, extremity acutely rounded ; inner 

 margin minutely crenulated. 



ARCOPERNA, Conrad. 



Description. — Oval or oblong, inflated ; beaks terminal ; 

 hinge edentulous; ligament internal; muscular impressions 

 marginal. 



A. FiLOSA, nob. — pi. 10, fig. 14. 



Description. — Suboval, inflated, thin, pearly ; radiated with 

 minute, closely-arranged lines ; disk somewhat flattened 

 behind the umbonal slope ; posterior margin subtruncated 

 above, extremity rounded ; basal margin rounded posteriorly ; 

 beak terminal. 



This genus appears to me quite distinct from Modiola, and 

 is characteristic of the Eocene period. M. radiolata, Desh., 

 is congeneric, and the peculiarities of the species described by 

 Deshayes are, I think, of generic value. 



PECTEN, Lin. 

 Subgenus EBURNEOPECTEN, Conrad. 



Description. — Smooth, polished, thin, of an ivory -like 

 substance. 



This subgenus is common in Eocene strata, but I have not 

 met with it in later formations. 



P. sciNTiLLATUS, nob. — pL 10, fig. 4. 



Description. — Ovate, very thin in substance ; umbo ventri- 

 cose, narrow, apex acute; anteriorly the larger valve is 

 marked with minute fine lines, having a shagreen-like cha- 

 racter. 



The smaller valve of this species is unknown. 



OSTREA, Lin. 

 O. FALCIFORMIS, nob. — pi. 11, fig. 1. 



Description. — Subfalcate ; larger valve, with divaricating, 

 prominent ribs; posterior submargin depressed, and finely 

 ribbed ; upper valve convex, without ribs. 



A variety of this species has verj^ numerous narrow ribs, 



