272 



MOLLUSCA. CEPHALOPODA 



bilious small or absent. Septal necks short, directed backwards but 

 usually also with a small part projecting forwards. External lobe 

 divided by a small saddle ; external saddle narrow ; lateral lobe 

 angular and deep ; lateral saddle broad, rounded, and undivided. 

 Carboniferous. Ex. G. sphcericum, G. crenistria, Carboniferous 

 Limestone. 



Gastrioceras. Shell with longitudinal striae, often with 

 transverse ribs ; with tubercles at the margin of the umbilicus. 

 External margin broad, rounded. External lobe broad and deep, 

 divided by a saddle ; first lateral lobe deep, tongue-shaped, angular ; 

 second lateral lobe small, angular ; saddles rounded. Carboniferous 

 and Permo-Carboniferous. Ex. G. carbo?iarium, Coal Measures. 



Prolecanites. Shell smooth or striated, flattened, with a 

 large umbilicus. Lobes and 

 saddles numerous. External lobe 

 not divided ; two or three lateral 

 lobes, sharp ; lateral saddles nar- 

 row and rounded. Devonian and 

 Carboniferous. Ex. P. compressus 

 ( = henslowi), Carboniferous Lime- 

 stone. 



Ceratites (figs. 107 B, ill). 

 Shell discoidal ; on the sides are 

 ribs which often bear tubercles 

 near the umbilical and external 

 margins ; external margin broad, 

 convex or flattened ; umbilicus 

 large ; body -chamber short. 

 Saddles rounded, lobes denticu- 

 late ; external lobe broad and 

 short. Trias, especially Muschelkalk. Ex. C. nodosus, Muschelkalk. 



Trachyceras ' . Shell flattened, highly ornamented with ribs 



1 This and the following genera, which are coiled in a plane spiral, 

 with external siphuncle, and septal necks usually directed forwards in 

 the adult, were formerly regarded as constituting a single genus — 

 Ammonites. The genera now adopted are founded mainly on the form 

 and ornamentation of the shell, the character of the sutures, and the 

 length of the body-chamber. 



Fig. 111. Ceratites nodosus, from 

 the Muschelkalk. The shell 

 has been removed, exposing 

 the sutures, x 4. 



