HAMMATOCERAS, PELKCOOERAS. 231 



smooth on the first or second wliorl, the tubercles begin on 

 either the second or third whorl, and, gradually' dividing, spread 

 themselves out upon the abdomen as liifurcated pilffi, which dis- 

 appear on the borders of the channels. The keel makes its ap- 

 pearance at an early stage, probably on the second whorl, but 

 the channels are not visible until a much later period, and dis- 

 appear in the adult. Abdominal lobe broad and deep. Supei-ior 

 lateral broader, but of about the same depth ; inferior lateral 

 very shallow. Superior and inferior lateral cells equally divided ; 

 both are short, broad, and but slightly indented by the minor 



lobes. 



Middle and Upper Lias. 



Hammatoceras. Abdomen may be either rounded or acute, 

 alwaj's keeled, but never sulcated. Pilfe are prominent and 

 straight. Envelopment may extend over one-half the sides, or 

 only cover the abdomen of each internal whorl. The young de- 

 velop as in Phymatoceras, but are generally much broader ; the 

 pilse, also, do not become prominent so soon. Nor do they in- 

 variably begin by the development of tubercles on the sides, but 

 may make their appearance as fine, raised lines, and afterwards 

 become tuberculated. During the earlier stages of growth the 

 different species have a very close resemblance to the adult 

 Macrocephali. The lobes are more complicated than in Phyma- 

 toceras. Abdominal lobe broad and deep, and continued into 

 two long, narrow, minor lobes. Superior lateral narrower than 

 the abdominal. Inferior lateral hardly wider than the minor 

 lobes of the superior lateral, and of about the same depth. Ab- 

 dominal cell blunt. Superior lateral and inferior lateral very 

 narrow and deeply indented by the minor lobes. 



Middle and Upper Lias. 

 H. iNSiGNis, Schloth. T. 108, figs. 624, 625. 



Pelecoceras. Having but one species of this genus, it would 

 be exceedingly hazardous to give the generic characters. They 

 will, however, probably be found to be distinguished b}' the pe- 

 culiarly pointed aspect, shallowness and breadth of the lobes 

 and cells ; the limits of the envelopment, which last is greater 

 than in other genera of this family ; the acute form of the back, 



and the breadth of the whorls. 



Upper Lias. 



