DESCRIPTION OF SHELLS. 191 
by minute, perpendicular, tortuous lamine ; the posterior deeply-arched edges 
of the thin plates recede from the pit as they are piled up, forming a sloping 
surface which is covered with a thin membrane ; the centre of the edge of the last 
plate is situated at one-third the length of the shell from the posterior extremity ; 
the calcareous lining of the horny membrane has a radiated structure at the 
posterior part, which is also widely expanded and slightly reflected. The convex 
or dorsal surface of the horny membrane has a coat of fibrous carbonate of lime 
extended nearly to its margin; this coat is thickened and very rugged in the 
middle and about the apex or point over the small pit in the other surface ; from 
this point a portion of it is produced in the form of a nearly cylindrical spine, 
beneath which it expands thinly over the membrane, and is radiated. To com- 
plete the structure, a series of membranes lining each other cap the posterior 
part of the dorsal surface enclosing the spine, but not pressing close to it nor to 
each other where they approach its base, which is consequently surrounded by a 
number of cavities*. The shell thus described is placed beneath the skin on the 
back of the animal, in the sac which also encloses the viscera ; it is secreted by 
the surface of the cavity in which it lies. I have been thus particular in descri- 
bing the shell of the Sepza, that I may the more easily point out how the fossils 
before us differ from it, and show that however imperfect these remains are, they 
indicate the existence of parts not known in Sepia, and of sufficient importance to 
mark a group generically distinct. The general form may be considered the same 
in both, but the fossils are much deeper and narrower: the anterior extremity of 
the fossil is unknown ; the posterior portion is compressed laterally, and rendered 
smooth as if by the action of some large muscles such as do not exist in Sepia ; 
hence a rounded, dorsal ridge is formed (the extremity of which is called a callus 
by Deshayes), which rises either perpendicularly or obliquely above the base of 
the spine. The expanded posterior extremity (called by Deshayes the ventral 
lamina) of the principal plate or membrane is thick and toothed ; the spine (ros- 
trum or apophysis of Deshayes) is large, thick, and complicated in its structure ; 
these circumstances altogether show that great strength was required in these 
parts by the habits of the animal. On each side near thick prolongations of 
the base of the spine are several irregular depressions, beneath which the struc- 
ture (as shown in Tab. IX. fig. 17 at h) is longitudinally fibrous; these are 
probably marks of the attachment of powerful muscles, and if so, the shell was 
not loose in the mantle as it is in Sepia: and we would venture to suggest that 
the lateral fins of the animal may have been placed very low down the sides of 
* Buckland’s Bridgewater Treatise, tab. 44. f. 4 a. 
2c2 
