Royal Society. 465 



bag of the Belemnite, which was published in the Zoological Trans- 

 actions, vol. ii., and in the Cyclopaedia of Anatomy and Physiology 

 (Art. Cephalopoda). This discovery led him, on the strength of 

 deductions from the physiological relations of this organ, to re- 

 move the Belemnite from the Polythalamacea of De Blainville, and 

 place it in the higher order of the naked Cephalopods. 



The structure of the shell is next discussed, and the spathose dart, 

 or guard, is proved to be the result of original organization, both 

 by its microscopic structure and by the fact that the chambers of 

 the phragmocone have not been infiltrated by mineral substance in 

 any of the specimens described : the name phragmocone being applied 

 to the chambered and siphonated conical division of the compound 

 shell of the Belemnite ; and the term alveolus being restricted, in 

 the present paper, to the socket or cavity at the base of the guard, 

 in which the phragmocone is lodged. A detailed description is given 

 of the sheath of the phragmocone and of the structure of the cham- 

 bers. The state of preservation of the present specimens has enabled 

 the author to describe the form and extent of the mantle — its con- 

 tinuation over the exterior of the shell, and the arrangement of its 

 muscular fibres. The animal is provided with two lateral fins of a 

 semi-oval figure, which are attached to the middle of the mantle, in 

 advance of the spathose dart. 



The muscular fibres of the fins, the infundibulum and its muscles 

 are next described ; and also the head, the eyes, which are large and 

 sessile, and the cephalic arms, which are eight in number ; together 

 with traces of two slender superadded tentacala. The ordinary arms 

 are furnished with a double alternate row of sharp horny hooks, as in 

 some existing species of Onychoteuthis, but the arms are relatively 

 longer. Their muscular structure is traced in the fossil specimens, 

 and compared with that in the recent Decapoda. The ultimate, or 

 primitive fibres of the muscles of the Belemnite agree in size with 

 those in the Onychoteuthis ; but the character of the transverse striae, 

 which is feebly developed in the primitive muscular fibre of the Ce- 

 phalopods, is not preserved in the fossil. Of the interior organs 

 of the Belemnite, besides the ink-bag and duct, which had been be- 

 fore discovered by Drs. Buckland and Agassiz, the remains of the 

 horny lining of the gizzard are preserved in the present fossils. 



Thus the deduction that the higher, or dibranchiate type of Ce- 

 phalopodal organization is necessarily associated with the presence 

 of the atramental apparatus, is established by the demonstration, in 

 these fossil Belemnites, of a fleshy mantle, inclosing the shell, and 

 provided with a pair of muscular fins, of large and sessile eyes, and 

 of few, but large and complex cephalic arms. 



The author concludes by pointing out the more immediate affini- 

 ties of the Belemnites, and showing that it combines characteristics 

 which are now divided amongst distinct genera : as, for example, 

 first", a complex internal shell, divisible into the same principal parts 

 as that of the Sepia, but one of which has, secondly, the same essen- 

 tial chambered structure as the shell of the Spirula ; thirdly, unci- 

 nated cephalic arms, as in the Onychoteuthis ; and lastly, an ad- 

 vanced position of rounded fins, as in the Spirula and Rossia. 



Phil. Mas. S. 3. Vol. 24. No. 1 6 1 . June 1844. 2 H 



