AM MO NO IDEA. 



865 



numerous species are known, none of which transcends the limits of 

 the Jurassic system. Familiar forms are the H. (Hildoceras) bifrons 



Fig. 790. — Harpoceras {Hildoceras) bifrons. Lias, 



Allied to Harpo- 

 shell is involute, 



(fig. 790) and H. serpentinum of the Upper Lias. 

 ceras is the Jurassic genus Oppelia, in which the 

 and the umbilicus is much reduced in 

 size. 



Family 14. Haploceratid^e. — In this 

 family the shell is discoidal, with a wide 

 or narrow umbilicus ; the external margin 

 being rounded, or in a few cases feebly 

 keeled. The surface is adorned with fine 

 lines of growth or curved ribs, commonly 

 with falciform constrictions or varices. 

 The aperture has feebly developed auricles; 

 the suture-line is deeply incised ; and an 

 " Aptychus " has hitherto been recognised 

 in a few forms only. 



The family of the Haploceratidce ranges 

 from the Lower Jurassic rocks to the Chalk, and the principal 

 genera are Haploceras and Desmoceras (fig. 791). 



Family 15. Stephanoceratid^e. — The form 

 family is variable, but the external margin is in 

 rounded, and is never keeled ; while the surface is almost always 

 adorned with transverse ribs, which may carry tubercles, and usually 

 bifurcate near the ventral border (fig. 792). The body-chamber 

 occupies from one-half to two-thirds of the last volution. The 

 aperture is commonly furnished with wide lateral extensions or lap- 

 pets, which may be inflected to form a sort of cowl (fig. 763). The 



vol. 1. 3 1 



Fig. 791. — Desmoceras {Am- 

 monites) ligatum. Cretaceous 

 (Neocomian). 



of the shell in this 

 general broad and 



