460 THE GEOLOGIST. 
the group; but it would be very important to discover whether the 
spiral processes are similarly disposed in the Athyris concentrica, 
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 Athyris. It is, however, certain 
that in t'je young condition Merista was possessed of a circular aperture 
for the passage of a peduncle, which became obliterated at a more 
advanced age from the incurvation of the beak. The interior of the 
larger -v alve differs from 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 Athyris 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 sheU-structure of both Athyris and Merista has 
been found to be impunctate.* 
The term Eetzia 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 shell-structure is 
punctate ; but further research will 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 
similar 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 adjuster muscles, for Mr. Hancock has 
observed that " in Waldheimia cranium, and in Terehratulina caput -serpent is, the 
dorsal adjuster muscles are not attached to a hinge-plate, but are inserted into the 
valve itself, and that in a species differing but little from W. Auslralis, the 
divaricators and accessory dlvaricators are united." 
