CEPHALOPODA. 27 
spathose guard of the Belemnite. It has a tendency to split in two along the centre, 
in a vertical plane, from the ventral to the dorsal aspect; and it is composed of 
successive conical layers, each enveloping the preceding layer, and exhibiting a fibrous 
texture crosswise. The anterior lateral and dorsal portions present straight plates, 
longitudinally fibrous, resembling the structure of the Beloptera Belemnitoidea, and the 
external edges hang over each other, and give an imbricated appearance to that part 
of the rostrum. 
In order to appreciate the differences which appear to render it desirable that the 
genus Belosepia should be retained, it may be well to give a short description of the 
internal shell of the recent Sepia officinalis. This will be found to consist of five 
distinct parts: Ist, an outer layer of calcareous matter, called the dwchler or sheath, 
convex, rugose externally, and prolonged at the posterior extremity into a calcareous 
spine, placed in the medial line, and inflected towards the ventral aspect; 2d, a series 
of horny layers imposed one over another, extending over the posterior dorsal surface 
of the buckler, and wholly enveloping and extending beyond the spine; 3d, a thin 
horny layer spread over the whole of the internal surface, and extending beyond the 
edges of the buckler, and which, in its turn, is entirely covered by, 4th, a calcareous 
layer, which contains the spongioid tissue and, 5th, a series of convex horny lamine, 
impregnated with carbonate of lime, placed horizontally, the posterior edge of each 
succeeding lamina being a little withdrawn from that of the preceding lamina, so that 
by this mode of superposition they present a depression or cavity immediately above 
the origin of the spine, and gradually rise into a convex mass at the middle and 
upper extremity of the shell. The spaces between the lamine act as air-chambers, 
but there is not any siphuncle or siphonal opening ; and the surfaces of the lamin 
are studded with an infinite number of minute columnar and sinuous partitions, placed 
at right angles to the lamin, and giving them support. 
It will be seen from this that the Belosepion, although bearing a close general 
resemblance to the Sepion, still presents several strongly-defined differences. The 
elevated calcareous mass or callus, which, in the Belosepion, terminates the sheath on 
the dorsal aspect, attaining frequently a considerable size, is not found in the Sepion ; 
and the fold, which in the latter is represented by a series of horny layers, distinct 
from, but wholly enveloping, the spine, is, in the former, a thick calcareous plate, 
formed by the expansion and retroflection of the anterior extremity of the rostrum, and 
extending barely beyond the line of the callus. In the Sepion the rostrum is small, in 
some species little more than rudimentary, and inflected, if at all, towards the ventral 
aspect. In the Belosepion, on the contrary, it attains a very large size, and, as M. Pictet 
observes, would indicate a gigantic animal if it were in relation to the animal in the 
same proportion as the Sepion; and it is invariably inflected towards the dorsal aspect. 
The internal lamine of the Sepion are horizontal, equidistant, and parallel, and so 
arranged as to form a hollow at the posterior ventral portion of the sheath, but rising 
