136 



shows similar strong rounded and blunt dental lamellae, 

 with a deep rostral cavity and muscular markings, which 

 would give a cast similar to that of C. glans-fagea. 



The cast of the dorsal valve of C. glans-fagea presents a 

 slightly concave surface, and on each side of the apex two 

 large and deep cavities made by the bases of the crural pro- 

 cesses ; and between them is a narrow filling of stone. 

 The centre is marked by a double muscular impression, the 

 two parts separated by a narrow groove : above this, and at 

 the base of the crura, are some points marked as if for 

 muscular attachment (see b, fig. 16.) 



The interior of C. impressa presents a very strong double 

 process below the beak of the dorsal valve, corresponding 

 to those in C. glans-fagea. 



The external form of all the species heretofore referred 

 to Cbntronella is a distinguishing feature, and, when ac- 

 companied by an internal apparatus so different from that 

 of Cryptonella, will serve to separate them from all the 

 allied forms. 



As before remarked, it has been mainly upon modifications 

 of this internal loop, or the apohysary system, that the 

 separation of most of the genera in the family of Tere- 

 bratulid^e has been made. 



In Cryptonella, we observe considerable analogy with 

 Renssel^ria, where the slender bands of the loop expand 

 and unite in a broad plate, which is obtusely or acutely at- 

 tenuate in front, and on the ventral side marked by a ridge 

 along the line of junction ; from which, at the posterior 

 margin, proceeds a slender process into the ventral cavity. 

 We may readily conceive of this central longitudinal ridge 

 or carina along the cicatrix of the two parts, being pro- 

 duced into a thin vertical plate, projecting backwards in 

 the line of the process from the base of the conjoined lainellse 

 in Renssel^eria, when it would much resemble the median 

 plate of Cryptonella (see figures 18, 19, 20 and 21). 



