PTEROPODA. 237 



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 

 the 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 internal one, and 

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

 through their centre. The interval between the two testaceous 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, Lamarck, 



Commonly called Nummulites, 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. 



CLASS II. 



PTEROPODA. 



THE Pteropoda, like the Cephalopoda, swim in the ocean, but being 

 deprived of feet, can neither fix themselves to other bodies, nor crawl. 

 Their organs of locomotion consist of fins placed like wings on the two 

 sides of the mouth. But few and small species are known. 



