CEPHALOPODA. 181 
and affords no clue to the many curious modifications observable 
in the fossil forms. Inthe 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 Pl. IEI., Fig. 5). 
In phragmoceras and gomphoceras (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withdrawn its head into the shell like the nautilus. 
Tig. 47. Gomphoceras Fig. 48. Phragmoceras.} 
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 (ss), which is almost isolated, may have 
* Fig. 45. Section of Ammonites obtusus, Sby. lias, Lyme Regis ; from a very young 
specimen. Fig. 46. Section of goniatites sphericus, Sby. carb. limestone, Bolland (in 
the cabinet of Mr. Tennant). The dotted lines indicate the lateral extent of the 
body-chamber. 
} Fig. 47. Gomphoceras Bohemicum (Barrande), reduced view of the aperture; s, 
the siphonal opening, Fig. 48. Phragmoceras callistoma (Barr.), both from the U, 
Silurian, Bohemia, 
