ASPIDOCERAS, 265 



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Genus HETEROCERAS, d'Orb. 



Heteroceras embraces a number of forms of very peculiar 

 slinpe, which stand in the same relation to Grioceras as Turri- 

 lites to Hamites. Our genus differs from Grioceras in departing 

 from the spiral coiled in one plane, from Turrilites 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'Orbigny. Besides the three typical species, Turrilites 

 Senequieri, d'Orb., is also to be placed here. 

 HETEROCERAS EMERTCII, d'Orb. T. 101, fig. 532. 



Genus ASPIDOCERAS, Zittel. 



External form very variable, sometimes flat 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. Sculpture consisting of one or two rows of tubercles or 

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

 Margin of aperture simple (Asp. aporum with ears?), body- 

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

 iiptyohi. Lobular line tolerabbr 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. 



The development of Aspidoceras is pretty well known ; the 

 branching off of Perisphinctes seems to take place in the upper 



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

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

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



Kard to the origin of the groups Perisphinctes aurigerus and 

 'vicosta; 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 double series of tuber- 

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

 34 



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