276 



MOLLUSCA. CEPHALOPODA 



Amaltheus. Shell flattened ; with a keel, which is usually 

 toothed but sometimes sharp ; umbilicus generally small ; surface 

 smooth, or with strias, or simple or spiny ribs. Aperture with a long 

 process at the external margin. Lobes and saddles deep and much 

 divided, with several auxiliary lobes. Jurassic. Ex. A. margari- 

 tatus, Middle Lias. 



Harpoceras (fig. 117). Shell flattened, with a prominent 

 even keel having a shallow furrow 

 on each side ; umbilicus small or 

 of moderate size. Sides of shell 

 with sickle-shaped undivided ribs. 

 Aperture with projections. Su- 

 tures strongly divided ; superior 

 lateral lobe deep. Lias. Ex. H. 

 serpentinum, Middle and Upper 

 Lias. 



Hildoceras. Similar to the 

 last. Whorls low, subquadrate in 



section, with broad external margin Fi S- n 7- Harpoceras serpentinum, 



, ,, , r , from the Upper Lias. x i. 



and usually a deep furrow on each 



side of the keel. Umbilicus wide. Ribs distinctly sickle-shaped 



in most cases. Upper Lias and base of Inferior Oolite. Ex. H. 



bifrons. 



Dactylioceras. Whorls numerous, only a little embracing, 

 without a keel ; umbilicus large. Ribs numerous, at first straight, 

 afterwards bifurcating, continued over the external margin ; without 

 tubercles. Body-chamber long. Sutures moderately divided ; ex- 

 ternal lobe larger than the superior lateral. Upper Lias and lower 

 part of Inferior Oolite. Ex. D. commune, Upper Lias. 



Stepheoceras ( = Stephanoceras) (fig. 118). Whorls thicker 

 than high, external margin rounded, without a keel. Umbilicus 

 large. Surface with straight ribs, which bifurcate on the sides of 

 the whorls and often bear tubercles where they bifurcate. Body- 

 chamber long. Aperture often with lobes. Sutures deeply divided ; 

 external lobe large ; inferior lateral lobe and auxiliary lobes small. 

 Inferior Oolite to Oxfordian. Ex. S. humphriesianum, Inferior 

 Oolite. 



