CEPHAIOPODA. 



181 



and affords no clue to tlie 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 gompJwceras (Figs. 47, 48) the aperture is 

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

 "withdrawn its head into the shell like the nautilus. 



Fig. 47. Gomphoceras 



Fig. 48. Plu-agmoceras.t 



M. Barrandc, 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 ohtusxis, Sby. lias, Ljnne Eegis ; from a very young 

 specimen. Fig. 46. Section of goniatites sphcericus, Sby. carb. limestone, Bolland (in 

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

 body-chamber. 



t Fig. 47. Gomphoceras Bohemicum (Ban-ande), reduced view of the aperture ; s, 

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

 Silurian, Bohemia. 



