Mr. J. S. Miller on Belemnites. 57 



cumstances of their structure ; namely, the Nautilacea and Sepiacea. The 

 former are provided with niultilocular shells, probably intended as an apparatus 

 to regulate the animal's buoyancy : but of this subdivision, only one genuSj 

 the Spirula, has been examined with anatomical precision^ and it therefore re- 

 mains involved in much obscurity. The second subdivision, the Sepiacea, have 

 no shell ; but in lieu of it, a complex laminated calcareous mass (somewhat im- 

 properly termed bone, and well known in commerce as cuttle-fish bone,) exists 

 in the genus Sepia, and a horny plate in the genus Loligo. The former may 

 very possibly, like the multilocular shells, assist, by its porous and cellular 

 structure, to render the animal more buoyant ; but the latter can scarcely 

 answer any other purpose than giving a support to the contiguous muscles. 

 The animal of the Belemnite probably formed an intermediate class of the 

 order, uniting the internal multilocular shell of the Spirula with the laminated 

 calcareous mass of the Sepia, to which the belemnitic guard appears to corre- 

 spond, as it is formed in a manner exactly similar ; namely, by the apposition 

 of successive laminae, of a concentric curvature, upon each other, in successive 

 strata : the only difference being, that in the cuttle-fish those spaces are not 

 closed and the edges of the laminae do not meet in a circle ; whereas in the 

 Belemnite, the parts secreting the shell, bending and closing round so as to 

 embrace a conical surface, made the laminae assume the same direction and 

 curvature. From the interstices between the laminae not having been empty, 

 but, as I have supposed, filled with spar at the period when the animal formed 

 the guard, I conjecture that the buoyancy of the animal must have been pro- 

 vided for by the chambered shell, and that the guard acted as a counterpoise 

 to it. 



With these views, I believe the inhabitant of the Belemnite to have been a 

 Sepia-like animal, having a body of an abbreviated form partly inserted into 

 the first chamber or aperture of the chambered cone, and connected to it by a 

 duct extending from it to the end of the siphuncle. I further conceive, that a 

 powerful circular muscle inclosed the chambered cone as far as the place where 

 the laminae of the guard encompass it; and that another muscle extended on 

 one side over the guard for its whole length, having laterally adhering to it 

 on each side, a muscle provided with a secreting surface and capable of en- 

 compassing the guard. This form of the animal would admit the formation 

 of the shell of the Belemnite, and correspond with the details its organization 

 presents. 



When the animal increased in size, it formed a new septum in front of the 

 chambered cone, adding at the same time a new rim to the cone. Hereby its 

 shell acquired a new chamber and a greater degree of buoyancy, to counteract 



VOL. II. SECOND SERIES. I 



