460 



THE GEOLOGIST. 



the group; but it would be very important to discoTer whether the 

 spii'al processes are similarly disposed in the Athyria conce7itrica, 

 A. Roissyi, and in some other species of the same genus, especially as in 

 the last two the interior characters of the shell have been ascertained to 

 be similar. 



The term Merista has been proposed by Professor Suess to distinguish 

 certain similarly- shaped species, but which exhibit some peculiar in- 

 terior dispositions not to be found in AtJiyris. It is, however, certain 

 that in t' .e young cocdition Jf^rts^a was possessed of a circular aperture 

 for the p.-issage of a peduncle, which became obliterated at a more 

 advanced age from the incurvation of the beak. The interior of the 

 larger valve difters fi^om that of Athyris in the presence of an arch- 

 shaped plate or process, which is situated between the dental plates, 

 and in the smaller valve by the absence of a regular hinge-plate, this 

 portion of the shell being divided by a narrow channel, from which a 

 large, deep, longitudinal septum originates, extending to about two- 

 thirds of the length of the shell, while in AtTiyris a small ridge is alone 

 observable. The spiral processes are fixed to the prolongations of the 

 socket-ridges, and have their extremities directed outwards, but the 

 exact shape of the connecting process has not hitherto been clearly 

 shown. It will therefore be very desirable that researches should be 

 continued until a complete restoration of all the interior parts can be 

 represented. The shell- structure of both Athyris and Merista has 

 been found to be impunctate.*^^ 



The term Retzia has been made use of by Professor King for the 

 reception of certain Terebratula-shaped shells, which possess a small 

 hinge- area, truncated beak, and circular foramen, which is partly com- 

 pleted by a distinct deltidium. The spiral appendages have their 

 extremities directed outwards, and the intimate sheH-structure is 

 punctate ; but fui'ther research wiU be required before the exact 

 interior dispositions and details can be correctly represented. 



* The muscular impressions in the interior of the larger valve of Merista are 

 similai' to those of Athyris, but we are not perfectly acquainted with those of the 

 other valve. The presence of a hinge-plate is not absolutely required for the 

 insertion of the dorsal pedicle or dorsal adjustor muscles, for Mr. Hancock has 

 observed that *' in WaMheimia cranium, and in Terelratulina ca^ict-serpentis, the 

 dorsal adjustor muscles are not attached to a hinge-plate, but are inserted into the 

 valve itself, and that in a species differing but little from TT. Ausiralis, the 

 divaricators and accessory divaricators are united." 



