Invertebrata. Zoological Collections. 373 



of that of the alveolus ; it has more or less distinct sinuses in these two 

 parts ; that of the dorsum is commonly the deepest. 



The successive line of the summits, generally called the axis, differs much 

 from the geometrical axis of the sheath. 1 call it the apicial line; it is 

 always eccentric, and closer to the venter than to the dorsum of the shell ; 

 and it is frequently arched. The summit of the sheaths often has very 

 deep grooves more or less prolonged, which are always placed symmetrically ; 

 their number is never very considerable, being 1, 2, 3, 5, and 7. There 

 are frequently, between these, other grooves in greater number, but much 

 more shallow ; I call these plaits, (plis : ) at other times they are still more 

 numerous and finer, and are nothing more than true striae. The summit 

 is sometimes terminated by a pore, or umbilicus, and when the successive 

 umbilici do not close, there remains an apicial canal instead of an apicial 

 line : the latter is quite independent of the siphon of the alveolus. The 

 surface of the sheath frequently has a canal, or an incision along the venter, 

 (the ventral canal,) beginning near the base, and extending towards the 

 summit. The apicial region is the part of the sheath which corresponds 

 to the apicial line, and the alveolar region that v/hich corresponds to the 

 alveolus. 



Of the specific characters of the Belemnites. — The specific characters of the 

 Belemnites are principally found in the ventral canal ; in the position and 

 form of the apicial line of the alveolus ; in the position, number, and length 

 of the latero-dorsal groove of the summit ; in the compression of the shell 

 between its sides, or the depression between the venter and the dorsum ; in 

 the general form, which is cylindrical, or conoid, or spear-shaped, or clavi- 

 form ; and, lastly, in the spherule, or globule, which sometimes terminates 

 the summit of the alveolus. The more or less rounded, or pointed form of 

 the summit, the ventral grooves, the plicae and striae which it frequently 

 presents, commonly serve only to divide the species into varieties ; as also 

 do the small variations in the degree of compression or depression of the 

 sheath, in the relation between the length of the apicial region and the 

 alveolar region, or in the angle of the ventro-dorsal profile of the alveolus. 



Of the growth of the Belemnites The membrane which secreted the 



sheath must have formed it successively, layer by layer, beginning evidently 

 by the less extended layers, that is to say, the interior, and ending by the 

 more extended, those which cover the others ; it was, therefore, necessarily 

 external to the shell. This fact was, indeed, indicated by the situation of 

 the striae of growth in the interior of the alveolar cavity, for the striae of 

 growth are found in all shells on the surface, opposite to the membrane 

 which secretes them. It follows from this, that the Belemnite was an 

 internal shell, like the os sepice ; there was, therefore, invariable contact 

 between the secreting organ and the secreted shell ; and, although the secre- 

 tion took place by enveloping layers, the exudation from each secreting pore 

 proceeded in a continuous line, and produced a fibre indicating the course 

 which the pore had followed during the whole growth of the animal ; and, 

 by this means, we may account for the fibrous structure of the sheath of the 

 Belemnites. 



The fibrous structure is more or less marked in all the bivalve shells, 

 because in these shells there is also an invariable contact between the shell 

 and the secreting organ. In the Inocerami and the Trichites, we see this 

 fact very plainly. The fibres are lamellar, and collected in groups, or the 

 laminae all belong to a triple system of planes, parallel to those of a calcare- 

 ous rhomboedron, the axis of which is parallel to the fibres. This crystal- 

 lization by groups is, perhaps, an effect of the petrifaction ; for, in the shells 

 of living Pinnce, it is not seen, as I have assured myself since the printing 

 of my work. The shell of these bivalves is composed of an assemblage of 

 horny tubes, the interstices of which are filled with carbonate of lime, so 

 that the earth of each fibre is not in contact with that of the neighbouring 



