864 



TETRABRANCHIATE CEPHALOPODS. 



Fig. 788. — /Egoceras capri- 

 cornum. Lias. 



the genus Oxynoticeras includes a number of Jurassic and Cre- 

 taceous " Ammonites," which differ from those comprised in 

 Amaltheus chiefly in the form of the suture-lines. 



Family 12. ^Egoceratid^. — The shell in this family of "Am- 

 monites " is discoidal, compressed, and usually widely umbilicated ; 

 the surface generally with simple transverse ribs. The body-chamber 



occupies about three-fourths of the last 

 volution, and the aperture is without 

 lateral extensions. The suture is incised, 

 with two lateral lobes, incompletely de- 

 veloped auxiliary lobes, and a bifurcated 

 antisiphonal lobe. An "Anaptychus" 

 is present (Zittel). 



The type -genus of this family is 

 ^lELgoceras (fig. 788), in which the sur- 

 face shows simple transverse ribs spread 

 out or split ventrally. The external side 

 of the shell is rounded, without either a 

 median keel or furrow. The species of 

 this genus are confined to the Lias. The 

 most important genus in this family is, 

 however, Arietites (fig. 789), in which the shell is flat and discoidal, 

 widely umbilicated, and many-whorled. The external margin is 

 more or less flattened, and is provided with a median keel bordered 



by a lateral furrow on each side. 

 The surface has simple straight ribs, 

 which often have tubercles developed 

 on them near the ventral margin. 

 The body-chamber is long (fig. 762), 

 and occupies one volution or more. 

 A large number of species are known, 

 all of which are confined to the 

 Lower Lias, A. Bucklandi, A. ob- 

 tusus, A. bisulcatus, and A. roti- 

 formis being familiar species. If 

 the name of Ammonites is to be employed at all as a generic 

 designation, it must be for the forms here in question. 



Family 13. Harpoceratid;E. — In this family the shell (fig. 790) 

 is discoidal, compressed, and umbilicated ; the external margin is 

 obtusely or acutely keeled ; and the surface is adorned with falciform 

 ribs or striae. The aperture is falciform, and is furnished with 

 rounded auricles and a long, pointed, ventral process. The suture 

 is digitated, and the antisiphonal lobe is undivided. A calcified 

 " Aptychus " is present. 



The type of this family is the genus Harpoceras, of which very 



Fig. 789. — Arietites {Ammonites) bisul- 

 catus, reduced in size. Lias. 



