CEPHALOPODA. 



181 



and affords no clue to the many curious modifications observable 

 in the fossil forms. In the ammonites we frequently find a dorsal 



Fig. 45. Ammonites, 



Fig. 46. Goniatites. 



process, or lateral projections, developed periodically, or only in 

 the adult (Fig. 62, and PI. III., Fig. 5). 



In phragmoceras and gomphoceras (Figs. 47, 4^) the aperture is 

 so much contracted that it is obvious the animal could not have 

 withdi'aTvn its head into the shell like the nautilus. 



Fig. 47. Gomplioceraa Fig. 48. Pliragmoceras.f 



M. Barrande, from whose great work on the Silurian Forma- 

 tions of Bohemia these figures are taken, suggests that the lower 

 part of the aperture (s s), which is almost isolated, may have 



* Fig. 45. Section of Ammonites obtusus, Sby. lias, Ljniie Eegis ; from a very young 

 specimen. Fig. 46. Section of goniatites sphcericus, Sby. caib. limestone, BoUand (in 

 the cabinet of Mr. Tennant). The dotted lines indicate the lateral extent of the 

 body-chamber. 



t Fig. 47. Gomphoceras Bohemicum (Barrande), reduced view of the aperture ; *, 

 the siphonal opening. Fig. 48. Phragmoceras callistoma (BaiT.), both from the U. 

 Silurian, Bohemia. 



