AMMONOIDEA. 



857 



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^k 





J" 



-Ceratites nodosus. 

 (Middle Trias). 



Muschelkalk 



The type of this family is the genus Ceratites (fig. 772), in which 

 the shell is discoidal, the whorls in contact, and the inner volutions 

 exposed to view. The lobes of the suture are denticulated or 

 crenulated, while the saddles are simply rounded. The genus is 

 characteristically Triassic, 

 and the majority of the 

 species are found in the 

 Muschelkalk. In the 

 Triassic genus Trachy- 

 ceras (fig. 773) the gen- 

 eral characters resemble 

 those of Ceratites, but the 

 saddles are in general 

 laterally incised instead 

 of being quite simple. 



The preceding genera 

 possess a discoidal shell, 

 the whorls of which are 

 in contact, but there are 

 three Triassic genera in- 

 cluded in this family in which the form of the shell is different. 

 Thus, in Choristoceras the shell forms a flat spiral, the inner whorls 

 of which are contiguous, while the last whorl is disjunct. In Cochlo- 

 ceras, again, the shell is turreted, while there is the additional 

 peculiarity that the lobes, 

 as well as the saddles, 

 of the suture are simple. 

 Lastly, in Rhabdoceras the 

 shell is straight, the lobes 

 of the suture being like- 

 wise free from denticula- 

 tions. 



Family 6. Cladisci- 

 TID.E. — In this family the 

 shell is discoidal, and the 

 body-chamber is long, ex- 

 tending over one whorl or 

 thereabouts. The shell is 

 thick and laterally flattened, the surface being spirally striated or 

 smooth. The suture shows foliaceous saddles and finely incised 

 lobes. This family was founded by Zittel to include certain Triassic 

 " Ammonites," most of which belong to the genus Cladiscites itself. 



Family 7. PinacoceratidjE. — The shell in this family has the 

 form of a compressed disc, usually with a smooth surface. The 

 body-chamber is short, occupying from a half to two-thirds of the 



Fig- 773. — Trachyceras Aon. Trias. 



