200 PELTOCERAS, SIMOCERAS. 



next one which loses wholly or partially the external row of 

 tubercles, as Asp. Tietzei and acanthomphalum, and from the 

 first form the species with a broad external furrow take their 

 origin, as Asp. pressulum, Knopi, Beckeri, hybonotum, 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 up 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. 



Aspidoceras reaches the highest point of its development in 

 the Kimmeridgian, and dies out in the Neocomian. 



Forty-eight species. Jurassic, Cretaceous. 

 ASPIDOCERAS LONGISPINUM, Sowb. T. 102, figs. 548, 540. 



Genus PELTOCERAS, Waagen. 



This genus was established by Waagen in a preliminary com- 

 munication upon the cephalopoda of the Jurassic of Cutch in 

 India ; 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 

 Perisphinctse with curved ribs, the stem-form here Peltoceras 

 annulare presents quite straight ribs. A difference between both 

 genera lies in the appearance of persistent ears in Peltoceras ; it 

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

 representatives appear in the upper Callovian, and in the upper 

 Oxfordian the 'genus already dies out with Pelt, bimammatum. 



Thirteen species. 

 PELTOCERAS ARDUENNENSE, d'Orb. T. 112, figs. 000. 007. 



Genus SIMOCERAS, Zittel. 



Shell very flat, discoidal, umbilicus wide, with numerous 

 whorls, which increase in thickness very slowly (except in the geo- 

 logically oldest forms) ; external side rounded or grooved ; sculp- 

 ture 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, nl least three- 



