26 EOCENE MOLLUSCA. 



horizontal. This arrangement of the laminae is well displayed in fig. l/i, Tab. I, drawn 

 from a specimen found at Sheppy, for the use of which I am indebted to Mr. Dixon, 

 to whom it belongs. The ventral margins of the laminae extend quite across the 

 connecting plate before mentioned ; and on each side, at a short distance from the 

 extremities, they expand into the lateral portions of the laminae, small projecting frag- 

 ments of which are sometimes still found adhering to the sides of the sheath. It is 

 evident from this that the opinion expressed by M. Voltz, that there existed in each of 

 the laminae an opening placed near the ventral margin, is correct. These openings 

 appear to have been of an elliptical form, with their shorter axes in a line from the 

 ventral to the dorsal surface, and were lined with an extremely thin calcareous sheath, 

 which extended throughout the whole series of the laminae, and of which portions are 

 frequently found adhering to the inner edges of the ventral margins and the lateral 

 fragments of the laminae. This sheath corresponds with the siphon of the Belemnites, 

 and is represented in the Sepion by the calcareous layer which, extending over the 

 posterior edges of the laminae, covers the entire surface of the last lamina, and it 

 presents, as M. Voltz states, an intermediate form between the narrow, straight siphon 

 of the Belemnites and the wide, open cavity of the Sepion. 



Whether the spaces between the laminae were filled with minute columnar partitions, 

 similar to those which characterise the Sepion, or whether they were simple air- 

 chambers, we have not at present any evidence to determine. The probability is, that 

 they were simply air-chambers ; for no trace whatever of any substance similar to that 

 termed the spongioid tissue of the Sepion has been found, which, had any such 

 substance existed, might reasonably have been expected ; and the true siphonal 

 structure, to which the Belosepion presents so close an approximation, is always 

 associated with simple air-chambers. The Belosepion, as its rostrum indicates, be- 

 longed to a Cephalopod eminently littoral in its habits, and the size, notwithstanding 

 the extraordinary development of the rostrum, leads us to believe that the animal was 

 not only smaller, but a less powerful swimmer, than the recent Sepia. We should 

 expect, therefore, to find in it some provision for buoyancy beyond that with which 

 the recent Sepia is furnished, not only for the purpose of increasing the swimming 

 power of the animal, but also as a compensation for the large and dense rostrum and 

 callus which characterise its remains. But if the interlaminar spaces were filled with 

 any substance resembling the spongioid tissue of the Sepion, the floating apparatus of 

 the Belosepion would be apparently inadequate to the wants of the animal. The form 

 and mode of superposition of the laminae, somewhat resembling the arrangement of 

 the septa in Spirulirostra, present a closer analogy with the phragmocone of the 

 Belemnites than with the plates of the Sepion. These considerations give additional 

 weight to the opinion of M. Voltz, founded on the appearance of what he terms the 

 " alveolar sutures," that the Belosepion was a camerated and siphoniferous shell. 



The rostrum of the Belosepion presents a structure analogous with that of the 



