.44 SIXTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



form a single lanceolate plate, which may be more or less atte- 

 nuate in front. These laminae of the loop, after becoming thus 

 conjoined and spreading laterally, are abruptly deflected in a 

 vertical plate along the median line, extending into the cavity of 

 the ventral valve, as shown in figure 2 ; while looking upon the 

 dorsal side of the loop, this vertical plate may sometimes be seen 

 projecting backwards between the bands of the loop, as well as 

 extending in front, as shown in fig. 12. 



Fig. 12. ' 



Cryptonella. 



In casts of the ventral valve, we find the marks of two thin 

 dental lamellas extending to a greater or less distance below the 

 beak. Along the median line in the ventral cast, there is usually 

 a narrow flattened space limited by a slender line ; and on each 

 side a less distinct narrow space, limited in the same manner. In 

 the cast of the dorsal valve, there is a median impressed line, and 

 two of less strength on each side of this. 



The species of this genus, known to me, are the Cryptonella [Centronella) 

 Julia, and those described as Terebratula in the Thirteenth Report on 

 the State Cabinet, and which in the Fourteenth Report were referred to 

 Cryptonella, viz. Cryptonella ( T.) rectirostra, C. ( T.) lens, C. ( 2\) 

 planirostra; and C. eximia of the Fifteenth Report, as well as a new 

 species from the Schoharie grit. 



The Terebratula lincklcBni, which has the external characters of Cryp- 

 tonella, and which I have referred to that genus, presents some slight 

 differences in the muscular impressions, which, taken together with its ro- 

 tund form, are suggestive of true Terebratula, to which genus it may 

 possibly belong. 



The species of the G-enus Centronella, heretofore described, have the 

 ventral valve highly convex or subangular in the middle, with the dorsal 

 valve flattened or concave in the middle, or with a median depression and 

 convex at the sides. 



The characterji of the genus, as given in the descriptions and illustrations 

 of Mr. Billings, are as follows ; 



