46 SIXTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



in the cast of a ventral valve of C. glans-fagea, figure 15, we liave the 

 filling of a deep rostral cavity : the dental lamellae have been thick and 

 strong, not extending as thin plates into the cavity of the shell as shown in 

 several species of Cryptonella, but having a thick blunt termination which 

 leaves no space, or scarcely an appreciable one, to be filled between it and 

 the shell. The spur, or filling of the rostral cavity, is striated : at its base 

 in the centre, on the body of the cast, is a depression ; and on each side are 

 fainter striated impressions, indicating the points of muscular attachment. 



The interior of the ventral valve of Centronella impressa* 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 processes ; 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 apparently for muscular attachment ( See 6, 

 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 Centronella 

 is a distinguishing feature, and, when shown to be accompanied by an in- 

 ternal apparatus so diflferent 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 in- 

 ternal loop, or the apophysary system, that the separation of most of the 

 genera in the Family Terebratulid^ has been made. Through this for- 

 tunate discovery of Prof. Winchell, we are able in a perfectly satisfactory 

 manner to place these forms, referred by me to Cryptonella, in their 

 true relations to other genera, which were before inferred from the punctate 

 structure, peculiarity of foramen, deltidial plates, and muscular impres- 

 sions. 



In Cryptonella we observe considerable analogy with Renssel^eria, 

 where the slender bands of the loop expand and unite in a broad plate, 

 which is obtusely or acutely attenuate 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 produced into a thin vertical plate, and projecting 

 backwards in the line of the process from the base of the conjoined lamellaa 

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

 Cryptonella (See figures 18, 19, 20 & 21). 



* A very distinct species from C. hecate ( Billings ) of the Oriskany sandstone, 

 wMcft diflfers only in size from Centronella (Rhynchonella?) alveata (Hall), Tenth 

 Report on the State Cabinet, 1857. 



