-234 JtlLDOCERAS. (1RAMMOOERAS. ETC. 



Family HILBOCER ATID^E. (Includes all the Falciferi proper 



with smooth pibv.) 



HILDOCERAS. Abdomen keeled and channeled. Ribs large 

 rind broad. The young continue smooth throughout the first 

 whorl. Ribs, keel and* channels appear on the second whorl. 

 The ribs are not preceded by a line of tubercles, but begin as 

 folds, bent much in the same way as in the adult, but with the 

 abdominal bend inclined more towards the apex. The abdominal 

 lobe is shallow and broad. Superior lateral much deeper than 

 either the abdominal or inferior lateral lobes, the last named very 



narrow and shallow, minor lobes small and pointed. 



Upper Lias. 

 H. BIFRONS, Brug. T. 103, fig. 556. 



GRAMMOCERAS. Abdomen keeled, but not channeled. Whorls 

 flattened laterally, giving a discoidal aspect to the shells. Ribs 

 finer and less prominent than those of Hildoceras. The young 

 also continue smooth much longer, and channels never appear ; 

 they take, however, the same rounded form of the whorl. Septa 

 differ but slightly from Hildoceras in the higher species, and not 



all generically in the lower species. 



Upper Lias. 

 G. SERPENTINUS, Schloth. T. 103, figs. 558, 5;V.l. 



LEIOCERAS. Abdomen keeled, acute. Sides of the whorls 

 flattened. Envelopment uniformly greater than in Grammoceras. 

 The young differ, however, in being much flatter at the corre- 

 sponding periods of growth. The lobes and cells, also, are less 



obtuse, shallower, and much more numerous. 



Upper Lias. 



LEIOCERAS COMTLANATTS, Brug. T. 110, figs. 044, 645. 



Mr. Hyatt has since published the following additional families 

 and genera : I cannot satisfactorily intercalate all of them with 

 the foregoing scheme of classification and have therefore pre- 

 ferred to insert them here, all together. 



Family TU ACII Y<M-:i! ATI !>J<:. 



< i YMNOTOCERAS, Hyatt. The development of Ammonites Blakei, 

 Gabb, and the characters of its abdomen, separate it at once 

 most decidedly from any species of Trachyceras. The devel- 

 opment generally of a keel, or, in some varieties, of a raised 



