Eighty-eight species of Belemnites have already been
discovered;[1] and the vast numerical amount to which individuals
of these species were extended, is proved by the
myriads of their fossil remains that till the Oolitic and Cretaceous
formations. When we recollect that throughout
both these great formations, the still more numerous extinct
family of Ammonites is co-extensive with the Belemnites;
and that each species of Ammonite exhibits also contrivances,
more complex and perfect than those retained in
the few existing cognate genera of Cephalopods; we cannot
but infer that these extinct families filled a larger space, and
performed more important functions among the inhabitants
calcareous plates, alternating with horny plates, which form the shield and shallow cup of the Sepiostaire, (Pl. 44', Fig. 2, e. and Fig. 5. e.,) represent the hollow fibro-calcareous cone or cup of the Belemnite, surrounding its alveolus.
The margin of the horny plates, interposed between the calcareous plates of the shield and cup of the Sepiostaire, (Pl. 44', Fig. 4, e, e', e″, e.,) represents the horny marginal cavity of the cone of the Belemnite, beyond the base of its hollow calcareous cone, (Pl. 44' Fig. 7, e. e'. e″.) This horny sheath of the Belemnite was probably formed by the prolongation of the horny laminæ which were interposed between its successive cones of fibro-calcareous matter.
The chambered alveolus of the Belemnite is represented by the congeries of thin transverse plates, (Pl. 44', Fig. 4, b.) which occupy the interior of the shallow cup of Sepiostaire, (e. e'.); these plates are composed of horny matter, penetrated with carbonate of lime.
The hollow spaces between them, (Fig. 5, b, b',) which are nearly a hundred in number in the full grown animal, act as air-chambers to make the entire shell permanently lighter than water; but there is no siphuncle to vary the specific gravity of this shell; and the thin chambers between its transverse plates are studded with an infinity of minute columnar, and sinuous partitions, planted at right angles to the plates, and giving them support. (Fig. 6', 6″, 6‴.)
The absence of a siphuncle renders the Sepiostaire an organ of more simple structure, and of lower office, than the more compound shell of Belemnite.
- ↑ (See index to M. Brochant de Villiers' Translation of De la Beche's Manual of Geology.)
