10 AMMONITIDiE. 



Body-chamber short, one-half to two-thirds of a whorl long. 

 The sculpture on the convex portion is interrupted ; in the geo- 

 logically younger forms a more or less deep median furrow is 

 simken in, at which the ribs terminate in a tubercle. Aperture 

 with a short lobate process on the convex portion. Lobes agree- 

 ing with Tropites ; much simpler in the geologically older forms. 



AEPADiTES, Mojs., 1819. Periphery with a deep groove, some- 

 times bordered by smooth or nodulous carinations ; ribs numer- 

 ous, dichotomous from an umbilical nodosity. The older forms 

 have entire saddles ; the more recent ones are toothed to the 

 summit. T. Arpadis, Mojs. 



HERACLITES, Mojs., 18T9. Body-chamber only occupying half 

 a whorl ; ribs strong, nodulous on the sides ; periphery of the 

 last whorl flattened, with two thread-like spiral lines ; lobes 

 distinguished by several irregular notches, but deeply truncate. 

 T. Poschli^ Hauer. 



SAGENiTES, Mojs., 1819. Body-chamber occupying half or three- 

 quarters of the whorl ; ribs usually not interrupted at the per- 

 iphery, crossed by very close spii-al lines ; saddles high and 

 wide, branched, foliaceous; lobes branched; auxiliary lobes very 

 small. T. Giebelij Hauer. 



Gymnotooeras, Hyatt, 1811. 



Distr. — G. rotelliforme^ Meek (xxxvi, 90, 91). Trias ; Nevada. 



The development of Ammonites Blakei, Gabb, and the char- 

 acters of its periphery, separate it at once most decidedly from 

 any species of Trachyceras. The development generally of a 

 keel, or, in some varieties, of a raised periphery, over which the 

 piloe do not pass, shows that this is a different genus, character- 

 ized by a different mode of development. The septa are quite 

 similar to those of Trachyceras, but it is A'ery evident that in 

 the Trachycerae the septa cannot be looked to for generic differ- 

 ences. Great differences also occur in the amount of involution 

 of the different species and in the development of their external 

 characters. 



Glydonitse. 



Body-chamber short ; sutural line undulated ; lobes and saddles 

 simple, not dentate. 



Clydonites, Hauer, 1860. 



Etym.. — Kludon, the surge. 



Distr. — 21 sp. ; Upper Triassic ; Europe, Himalayas. 2 sp. ; 

 Upper Cretaceous (described by d'Orb. as Ceratites. Difference 

 from Ceratites is the lobes being simple, not crenulated). G. 

 costatus, Hauer (xxxii, 21). G. delphinocephalus, Hauer (xxxii, 

 28). 



Shell spiral, discoidal, whorls involute, ribbed ; sutures simply 

 lobed, the lobes pointed. 



