82 AMMONITID^. 



ceras, usually somewhat more deepl}^ notched, with a dependent 

 sutural lobe. Apt^^chus divided, calcareous, very thin, exter- 

 nally granular. 



This group corresponds to the Planulati of Buch. 



SiMOCERAS, Zittel. 



Distr. — 26 sp. Jurassic. S. Jooraensis, Waagen (xli, 66, 61). 



Shell ver}^ flat, discoidal, umbilicus wide, with nvimerous whorls, 

 which increase in thickness very slowly (except in the geologi- 

 cally oldest forms) ; external side rounded or grooved ; sculpture 

 seldom absent, consisting mostly of straight, simple or forked 

 ribs, which are interrupted during most of the lifetime of the 

 animal ; interrupted at any rate in the young state on the exter- 

 nal side, and which are often ornamented with tubercles or 

 strongly swollen on the last whorl; isolated constrictions directed 

 forwards on all the whorls. Body-chamber long, at least three- 

 quarters of a whorl, usually attaining a greater length. Lobular 

 line not very complicated. Siphonal lobe largest, external saddle 

 much developed and broad, laterals one-pointed, Yery small 

 in the geologically younger forms. Aptychus (?). 



REiNECKEiA, Bayle, 18*78. Shell close to Perisphinctes, but 

 with more marked constrictions, and a peripheral groove. S. 

 anceps^ Beinecke. 



Peltoceras, Waagen, 1871. 



Distr. — 13 sp. Jurassic ; Europe, India. P. Arduennense, 

 d'Orb. (xli, 64, 65). 



Shell when young having the ornamentation of Perisphinctes ; 

 when adult, with straight tuberculated ribs ; aperture with per- 

 sistent lateral projections. 



AspiDOCERAs, Zittel, 1866. 



Syn. — Waagenia, Neumayr, 1878. 



Distr. — 48 sp. Jurassic, Cretaceous. A. longispinimi, Sowb. 

 (xxxiv, 56, 58). 



External form very variable, sometimes flat with wide umbil- 

 icus, sometimes inflated with a narrow umbilicus ; external side 

 rounded or with a broad external furrow, never with a carina or 

 angle. Sculpture consisting of one or two rows of tubercles or 

 wanting. Bibs, as a rule, present only in the young state. Mar- 

 gin of aperture simple (Asp. aporiim with ears?), body-chamber 

 short, embracing two-thirds of a whorl. Lobular line tolerably 

 simple ; siphonal, two laterals, also often (in the geologically 

 younger species) an auxiliary lobe. Lobes not much cut (with 

 the exception of Asp. Altenense and circumspinosum) ; bodies of 

 the lobes and saddles broad. Cellulose aptychi. 



Aspidoceras reaches the highest point of its development in 

 the Kimmeridgian, and dies out in the Neocomian. 



