NUMBER OF SPECIES. 379 
Eighty-eight species of Belemnites have al- 
ready been discovered ;* and the vast numerical 
amount to which individuals of these species 
were extended, is proved by the myriads of their 
fossil remains that fill the Oolitic and Cretaceous 
formations. When we recollect that throughout 
both these great formations, the still more nume- 
rous extinct family of Ammonites is co-extensive 
with the Belemnites ; and that each species of 
Ammonite exhibits also contrivances, more com- 
plex and perfect than those retained in the few 
cone of the Belemnite, beyond the base of its hollow calcareous 
cone, (PI. 44', Fig. 7, e. e'. e''). This horny sheath of the Be- 
lemnite was probably formed by the prolongation of the horny 
laminae 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, (PI. 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 car- 
bonate 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. BrochanK. de Villiers' Translation of De la 
Beche's Manual of Geology). 
