NATURAL SCIENCES OP PHILADELPHIA. 289 



which differs essentially from Erycinelli, whilst the exterior is very much like 

 the latter. (Recent.) 



CRASSATELLA, Lam. 



C. producta. Trapezoidal, elongated, inequilateral, concentrically 

 ribbed ; ribs or ridges subacute, prominent as far as the umbonal slope which 

 is obtusely subcarinated, and curved inwards ; posterior slope depressed and 

 medially subangular, concentric lines not prominent except behind the umbo, 

 where they are more distinct; posterior extremity obliquely truncated; inner 

 margin minutely crenulated. 



Locality. Enterprise, Clark Co., Miss. (Eocene.) 



This elegant species may be distinguished from C. protexta by the furrows 

 or ridges extending over the whole disk. It belongs to a group of fossils quite 

 distinct from those of Claiborne, Jackson or Vicksburg. 



TRIGONIID.E. 



VERTICORDIA, Wood. 



V. Emmonsii, C, Emmons, Geol. N. C, 286 206. 



This is the only species yet known in an American formation. 



ARCIDuE. 



CUCULL^IA, Lam. 



The typical form of hinge teeth in this genus is not represented in the 

 Eocene or Cretaceous Arcidcv. The oldest form which could be referred to 

 Cucullsea occurs in the latter formation, represented by thick ponderous 

 shells, with a broad hinge, and are in marked contrast to that of the recent spe- 

 cies, whilst the character of the teeth is equally remote. I propose, therefore, 

 to characterize the cretaceous subgenus as follows : 



IDONEARCA, Conrad. 



Triangular, thick, delicately lined ; hinge thick, medial cardinal plates 

 transverse, laminar, laterally striated ; anterior and posterior plaits elongated, 

 oblique, curved downwards at the ends towards the umbo, and laterally striated; 

 interior plate curved, very prominent. 



Cucullcea Tippana, C, and T. capax, C. (Cretaceous.) 



The following European species will come under this section : 



Area tumida, d'Orbig., A. Marceana, d'Orbig., A. fibrosa, d'Orbig. 



TRIGONARCA, Conrad. 



Equivalve, trigonal, angular on the umbonal slope ; hinge area narrow ; 

 series of cardinal plates curved and placed transversely or nearly at right 

 angles to the hinge line. 



T. (Cucullcea) Maconensis, C, Journ. A. N. S. 



Viewed exteriorly, this shell has the habit of Cucullsea, but the hinge of 

 Axinsea. It probably has the internal elevated plate, but the specimens are 

 filled with stone too hard to be removed, (Cretaceous.) 



Subgenus Latiarca, Conrad. 



Triangular, thick, capacious ; hinge line narrow medially, broad and thick 

 on the sides ; cardinal plates granular and laterally striated, towards the ends 

 in short oblique series ; cardinal area wide with obliquely diverging grooves. 



L. (Cucullcea) gigantea, C, L. idonea, C, C. ononckela, Rogers, C. trans- 

 versa, Rogers. (Eocene.) 



ANOMOLOCARDIA, Klein. 



A. tr i gi nt i na ri a. Elongated, rhomboidal, very unequilateral, veil- 



1862.] 20 



