CEPHALOPODA 81 



dense muscular tissue, even in the same matrix which has pre- 

 served so completely the mummy cuttle-fish. By their form 

 and size they were ill adapted for rapid locomotion, and must 

 have depended for safety on the shelter afforded by their solid 

 shell. The discoidal Ammonites attained a diameter approach- 

 ing 3 feet, and the straight-shelled Orthocerata were sometimes 

 not less than 6 feet in length. These latter must have lived 

 habitually in a position nearly vertical ; whilst the discoidal 

 genera would creep over the sea-bed with their air-chambers 

 above them, like a snail-shell reversed. The Ammonites ap- 

 pear 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 frecjuently sculptured on their 

 outer convex surface ; whilst their concavity exhibits only 

 lines of growth (fig. 21, 7). Special forms are associated, in all 

 localities, with particular species of ammonite ; and their size 

 is adapted exactly to the specimens in which they are found. 

 Calcareous 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. 21, s) have been called "Bhyncholites" (Palceo- 

 teuthis and Rynchotcuthis of d'Orbigny) ; whilst the lower man- 

 dibles constitute the genus Conehorhynclius of De Blainville 

 (fig. 21, 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 par- 

 titioned off internally by a nacreous lamina, indicating con- 



G 



