98 PALEONTOLOGY. 



the discoidal genera would creep over the sea-bed with 

 their air-chambers above them, like a snail-shell reversed. 

 The Ammonites appear to have been provided with an 

 operculum, more secure than the "hood" of the Nautilus, 

 composed, like it, of two elements, not, however, fibrous and 

 confluent, but calcified and united by a straight suture. 

 These opercula, which have been mistaken for bivalve shells, 

 have a porous structure altogether peculiar and are fre- 

 quently sculptured on their outer convex surface; whilst 

 their concavity exhibits only lines of growth (fig. 31, 7). 

 Special forms of Aptychus are associated, in all localities, 

 with particular species of ammonite ; and their size is adapted 

 exactly to the specimens in which they are found. Calca- 

 reous mandibles occur in all the secondary strata, but not, 

 hitherto, in such numbers or circumstances as to imply that 

 they belonged to any other genus beside the true Nautilus. 

 They are of two forms: those corresponding to the upper 

 mandible (fig. 31, s) have been called "Bhyncholites" (Palceo- 

 teuthis and Rynchoteuthis of D'Orbigny) ; whilst the lower 

 mandibles constitute the genus Conchorhynchus of De Blain- 

 ville (fig. 31, 9). The arms of the extinct Tetrabranchs may 

 have been organized like those of the Nautilus, but were 

 probably less numerous in the genera with slender shells, and 

 in those early forms with a small many-lobed aperture. The 

 length of the body-chamber is greatest when its diameter is 

 least ; and the prominent spines which ornament the exterior 

 are partitioned off internally by a nacreous lamina, indi- 

 cating considerable motion of the animal in its shell. When 

 the outer shell of the fossil is removed by decomposition, or 

 the hammer, the margins of the internal septa (or partitions 

 of the air-chambers) are exposed: these marginal lines are 

 called " sutures." 



The chambered shells may be divided into two principal 

 groups, viz., those with simple sutures, like the recent nauti- 



