AS1'1T>0CERAS. 265 



Genus HETEROCERAS, .rOil.. 

 Heteroceras ciiibraci'S a iniijil)cr oi" foi'iiis of very peculiar 

 shape, which stand in the same relation to Crioceras as Tarri- 

 lites to Hamitea. Oiii- genus (litters from Crioceras in departing 

 from the spiral coiled in one plane, from TurriliteH in its asym- 

 metrically divided lateral lobes, but besides this in its whole 

 habitus and its quite abnormal curvature, known from the figures 

 of d'Orbign}'. Besides the three typical species, Turrilites 

 Senequieri^ d'Orl)., is also to be placed here. 

 HETERorERAS Rmertoii, d'Orl). T. 101, fig. 532. 



(ienus ASPIDOCERAS, Zittel. 



External form very variable, sometimes tlat with wide umbili- 

 cus, sometimes inflated with a narrow umbilicus ; external side 

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

 angle. Scul})ture consisting of one or two rows of tubercles or 

 wanting. Ribs, as a rule, present onl}' in the young state. 

 Margin of apertux-e simple {Asp. aporum with ears?), body- 

 chamber short, embracing two-thirds of a whorl. Cellulose 

 aptychi. Lobular line tolerably simple ; sii)honal, two laterals, 

 also often (in the geologically younger s[)ecies) an auxiliarj' 

 lobe. Ijobes not much cut (with the exception of Asp. Altenense 

 and circumspinosum) ; bodies of the lobes and saddles broad. 



The development of Aspidoceras is pretty well known ; the 

 branching off of Perisphinctes seems to take place in the upper 

 Callovian. If one breaks away the outer whorls of one of the 

 simpler, geologically old types, for example Asp. perarmatum. 

 one finds within a kind of sculpture, which leaves no doubt in 

 regard to the origin of the groups Perisphinctes aurigerus and 

 curvicosta ; curved ribs and tubercles are identical in both, and 

 the last are developed into the external row of tubercles of Aspi- 

 doceras., amongst which the forms with but one external row of 

 tubercles represent the original type, from which the bi-tubercu- 

 late Perarmata are first developed, which in the young stages, 

 according to the stage of the ribs and tubercles, pass through a 

 second with only an external row of tubercles, then definitely 

 the third with two rows of tubercles. 



To the large series of Perarmata with doubh' series of tuber- 

 cles, whicli have no auxiliary lobe, several other series are allied ; 

 ;}4 



