374 Zoological Collections. Iiivertebrata. 



fibres ; if it were a crystalline system, it would probably bave been inde- 

 pendent of the neighbouring fibres. 



The formation of the Belemnites must have commenced by the summit 

 of the alveolus, or by its spherule, when there is one ; and, from that point 

 the alveolus and sheath must have grown simultfineously, in such a way, 

 that the extension of the base of the alveolus was always accompanied by 

 new layers, deposited on the surface of the sheath. This mode of viewing 

 the growth is opposed to that laid down by M. de Blainville, in his memoir 

 on the Belemnites, where he supposes that the growth of the shell had three 

 distinct periods : in the first, he considers that there was not yet any conical 

 cavity, and the horns which composed the sheath were visible at the very base 

 of the shell ; in the second, there was a more or less deep cavity, and the striae 

 of growth were visible in the interior ; and, in the third period, the cavity 

 obtained the partitions. 



Relations of the Belemnites unth the Spirulce The structure of the Belem- 

 nites has the greatest analogy to that of the Spirulce, the shell of which is 

 composed, 1st, Of a granulated cortex, which represents the sheath of the 

 Belemnites, and is entirely calcareous ; 2d, Of an internal shell, which 

 represents the alveolus, and is composed of a horny matter, penetrated with 

 carbonate of lime ; 3d, Of partitions, with siphonic appendages, exactly 

 resembling those of the Belemnites, and composed also of horny and 

 calcareous matter. 



Relations with the Bebpterce — The Belemnites have also a close analogy 

 with the Beloptera belemnoidea, (Blainville,) which forms the passage from 

 the Belemnites to the Beloptera sepioidea, (Blainville,) for which, I think, a 

 new genus should be made, and which I have named Belosepia Cuvieri. 

 This last has a rostrum, which represents the apicial region of the Belemnites, 

 and is also composed of concentric fibrous layers ; the other part represents 

 the alveolar region, and has, at the same time, the greatest analogy with the 

 bone or shell of the Sepiae, (Sepiostarium.) The siphon appears to be 

 extremely wide in the Belosepia. 



Relations with the Sepiostarium, — The sepiostarium is likewise formed on 

 the same plan ; the granulated crust of the shield represents the sheath, 

 which is also graiuilated in certain Belemnites, and in the Spirulie ,- it is 

 composed of pure calcareous matter, as in these latter ; below, there is a 

 horny membrane, which probably also existed in the Belemnites, between 

 the sheath and the alveolus : then comes a shell of horny and calcareous 

 matter, which represents the alveolus ; it has also its stria of growth at the 

 upper surface, as in the Belemnites and Spirulse ; then the mass, so 

 improperly called the spongy matter, which is composed of partitions 

 and cavities. The great cavity which is found in the interior of the 

 shell towards the rostrum, represents the siphon, which is much wider than 

 in the Belosepia ; it is placed on the ventral side, as in the Belosepia, the 

 Spirulee, and the Belemnites. These partitions are of a horny and calca- 

 reous matter, as in the Spirulse ; but they are distinguished by being sup- 

 ported by a number of small pillars, a disposition which was necessary on 

 account of the fragile nature of these partitions, and their great extent. 



Relations with the Actinocamax — The Actinocamax, a genus established by 

 Miller, diflfers from the Belemnites in having sheaths without alveoli, and 

 without any alveolar cavity ; the enveloping and fibrous layers which have 

 successively formed the shell, have extended it only at the summit, whilst 

 at the base they always rest one on the other. 



Zoological affinities — It results from this, that the different shells which 

 I have examined, are all constructed according to one plan of organization, 

 and that their affinities may be represented by the following table : — 



Spirula, LrrHuiTA ? Orthocera. 

 Shell partitioned, sheath indistinct, or imperceptible. 



