HARPOCERAS. 253 



Siphonal lobe almost as deep as wide ; the point of attachment 

 to the siphon is exactly in the middle of its depth ; the upper 

 lateral does not attain half of its depth and is at least as broad 

 as deep ; the lateral saddle is more elevated than all the others 

 and stands above the base of the upper lateral usually double as 

 high as the external saddle; the lower lateral lobe is much 

 broader than deep and the antisiphonal saddle so small, that it 

 does not attain half the height and width of the lateral saddle. 

 Antisiphonal lobe two-pointed. Corneous, simple anaptychus. 



Waageii says that the separation of Arietites and ^Egoceras 

 is difficult, and Hyatt observes that a genetic interrelationship 

 exists between the two. The first representatives appear in the 

 lowermost Lias, and according to our present understanding of 

 the genus they appear to have died out in the lower Lias, though 

 many forms appear, which at present referred to Harpoceras, in 

 reality belong to Arietites, as, for example, Harp. Algoviamum. 



Thirt3 T -eight species. 



ARIETITES OPHIDIOIDES, Orb. T. 107, figs. 607, 608. 

 " SCIPIONIANUS, Orb. T. 108, figs. 629, 630. 



" OBTUSUS, Sowb. T. 107, figs. 614, 615. 



KRIDION, Orb. T. 106, figs. 605, 606. 

 BISULCATUS, Brong. T. 103, fig. 557. 



Genus HARPOCERAS, Waagen. 



External form of the shell fariable, external side always cari- 

 nate or angular ; sculpture consisting of more or less distinct 

 sickle-like ribs. Margin of aperture sickle-shaped, or with ears, 

 with pointed external lobes ; body-chamber embracing one-half 

 to two-thirds of a whorl, carinate to the margin of the aperture. 

 Aptychus divided, thin, calcareous, with a thick, shelly laj'er, 

 more or less folded. 



Lobes mostly not deeply notched, always two lateral lobes and 

 almost always auxiliaries. Siphonal lobes ending in two diverg- 

 ing branches, usually shorter than the first lateral ; laterals not 

 divided into symmetrical halves. 



Sharp demarkation from the genus ^Egoceras is wanting, since 

 the forms from the group including JEg. arietiforme, Opp., fit as 

 well into the one as into the other ; the most recent ffarpoceras 

 is Harp. Zio from the upper Kimmeridgian. The present genus 



