248 MOLLUSCA. 



that is to say such as are divided by septa into several cavities; their inha- 

 bitants he supposed to be Cephalopoda. 



This genus is now divided into Spirula and Nautilus proper. 



THE BELEMNITES 



Probably belong to this family, but it is impossible to ascertain the fact, as 

 they are only found among fossils; every thing, however, proves them to 

 have been internal shells, thin and double, that is, composed of two cones 

 united at base, the inner one much shorter than the other, and divided into 

 chambers by parallel septa, which are concave on the side next to the base. 

 A siphon extends from the summit of the external cone to that of the in- 

 ternal one, and continues thence, sometimes along the margin of the septa 

 and sometimes through their centre. The interval between the two tes- 

 taceous cones is filled with a solid substance here composed of radiating 

 fibres, and there of self-involving conical layers, the base of each being on 

 the margin of one of the septa of the inner cone. 



Of all fossils Belemnites are the most abundant, particularly in chalk and 

 compact limestone. 



THE AMMONITES, 



Or the Cornua-Ammoni, also fossil, are distinguished from the Nautili by 

 their septa, which, instead of being plane or simply concave, are angular 

 and sometimes undulated, but most frequently slashed on the edge like the 

 leaf of an acanthus. The smallness of their last cell seems to indicate that, 

 like the Spirula, they were internal shells. They are very abundant in the 

 strata of secondary mountains, where they are found varying from the size 

 of a lentil to that of a coach wheel. Their subdivisions are based upon the 

 variation of their volutes and siphons. 



THE NUMMULITES, Lam. 



Commonly called Nummulites y lenticular stones, &c. are only found among 

 fossils, and present, externally, a lenticular figure without any apparent 

 opening, and a spiral cavity internally, divided by septa into numerous small 

 chambers, but without a siphon. They constitute the most universally dif- 

 fused of all fossils, forming entire chains of calcareous hills and immense 

 bodies of building stone. 



