ADDENDA. 421 



Rliyncliuspira snb^Iobosa. 



PLATE LXIII, FIGS. 23-25. 



Among the collections from the Schoharie grit, there are a few specimens 

 of Rhynchospira which have much the general appearance of R. globosa of 

 the Lower Helderberg group. They differ in having a smaller number of 

 stronger plications, with a single, more prominent one in the middle of the 

 dorsal valve, and a corresponding wider depression on the ventral valve ; 

 while in R. globosa there are one, two or three smaller plications forming 

 a kind of mesial sinus on one or both valves, which is a constant and 

 characteristic feature of the species, as also of all those of the Genus in the ' 

 Niagara and Lower Helderberg groups. 



This simple mesial plication, together with the globose form, will distin- 

 guish this from any described species of the Genus. 



Remarks upon the Character of the Crura and Loop of Terebratula and 

 Cryptonella illustrated in this volume. 



A careful examination of the interior structure of the Terebratuloid 

 shells, has shown sufficient ground for the separation of the Genus Crypto- 

 nella, and the external character of the shells would seem to afford the 

 necessary means of distinguishing the Genera. Among the species included 

 under the Genera Terebratula and Cryptonella, there are three distinct 

 forms of loop which are in some degree connected with an external differ- 

 ence in the form of the shell. 



The simplest form of loop observed is similar to that of Terebratula 

 proper, but with the anterior extension of the crura free, or not connected 

 by any visible extension of the calcified substance of the loop, the extremities 

 appearing as if broken. There may have been some connexion of the parts 

 by ligament or otherwise during the life of the animal. This form of loop, so 



[Pal-sontoloot IV.] 55 



