2r.fi PELTOCERAS. SIMOCERAS. 



next one which loses wholly or partially the external row of 

 tuherelcs. us Asp. Tietzei and acanthomphalum, and from the 

 first form the species with :i liro:nl external furrow take their 

 origin, as Asp. pressulum, Knopi, Beckeri, liybonotmn. etc. 

 Finally. the inflated forms of Cyclota are to be referred to the 

 Perarmata. which may easily, on account of their great thick- 

 ness. take ii}) an auxiliary lobe, and also, analogous to the slen- 

 der forms, gradually lose the outer, later the inner row of tuber- 

 cles, becoming quite smooth. 



Axpidoceras reaches the highest point of its development in 

 the Kimmeridgian, and dies out in the Neocomian. 



Forty-eight species. Jurassic, Cretaceous. 



Asi'IDocKHAS LONGISPINUM, Sowb. T. 102, figs. .~>4S. ."UN. 

 Genus PELTOCERAS, Waagen. 



This genus was established by Waagen in a preliminary com- 

 munication upon the cephalopods of the Jurassic of ditch in 

 I ndia ; it embraces, according to my understanding, forms, which 

 like Aspidoceras. branch off from Perisphinctes and develop 

 tuberculate ribs; but whilst Aspidoceras is to be traced to the 

 Perisphtnctse with curved ribs, the stem-form here Prl (<,/".< 

 annulare presents quite straight ribs. A ditferencc between both 

 genera lies in the appearance of persistent ears in /V//V-r/v/s ,- it 

 is of importance to know the aptychus of the latter. The oldest 

 representatives appear in the upper Callovian. and in the upper 

 Oxfordinn the genus already dies out with Pelt, bimamiiwttnu. 



Thirteen species. 

 PELTOCKRAS A KM I:\.\KNSK, d'Orb. T. 11*2. figs, ur.fi. fifiT. 



SIMOCERAS, Zittel. 



Shell very Hat. discoidal. umbilicus wide, with numerous 

 whorls, which increaM 1 in 1 hirkness very slowly (except in 1 he 

 logically oldest form- ) : external side rounded or grooved ; sculp- 

 ture -eldom a 1 tsent . consist in g mostly of straight . simple or forked 

 rib-, which are interrupted during most of the lifetime of the 

 animal : interrupted at any rate in the young slate on the exter- 

 nal side, and whieh are often ornamented with tubercles or 

 ittgly -\\olleii <n t lie last whorl ; isolated constrictions directed 

 forward- on all the whorls. IJody-chamlier long, at least three- 



