286 NUMBER OF SPECIES. 



Eighty-eight species of Belemnites have already been dis- 

 covered;* and the vast numerical amount to which in- 

 dividuals of these species were extended, is proved by the 

 myriads of their fossil remains that fill the Oolitic and Cre- 

 taceous 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 contri- 

 vances, 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, alternaling with horny plates, which form the shield and 

 shallow cup of the Sepioslaire, (Pi. 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, (Pi. 44', Fig. 4, e, e, e', e.,) repre- 

 sents the horny marginal cavity of the cone of the Belemnite, beyond the 

 base of its hollow calcareous cone, (Pi. 44' Fig. 7, e. e'. e".) This horny 

 sheath of the Belemnite was probably formed by the prolongation of the 

 homy 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 carbonate of lime. 



The hollow spaces between them, (Fig. 5, b, b',) whicli 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, jilanted at right angles to the plates, and giving them support. 

 (Fig. 6', 6", 6'".) 



The absence of a siphuncle render, the Sepiostaire an organ of more 

 simple structure, and of lower office, than the more compound shell of Be- 

 lemnite. 



• (See index to M. Brochant de Villiers' Translation of De la Heche's 

 Manual of Geology.) 



