CEPHALOPODA. 27 



spathosc guard of the Bclcmnite. 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 Belqptera 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 : 1st, an outer layer of calcareous matter, called the buckler 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 scries 

 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 laminae, 

 impregnated w r ith 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 succeeding lamina being a little withdrawn from that of the preceding lamina, so that 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 laminae act as air-chambers, 

 but there is not any siphuncle or siphonal opening ; and the surfaces of the laminae 

 are studded with an infinite number of minute columnar and sinuous partitions, placed 

 at right angles to the laminae, and giving them support. 



It will be seen from this that the Beloscpion, 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 laminae 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 



