866 



TETRABRANCHIATE CEPHALOPODS. 



lobes and saddles of the sutures are incised, and a calcified " Apty- 

 chus " is present. 



In Stephanoceras itself (792) the shell is discoidal and usually 

 widely umbilicated, and the surface is ornamented with strong ribs, 



Fig. 792. — Stephanoceras (Ammonites) Humphresiamnn. Inferior Oolite. 



which are simple internally, but become once or more divided on 

 approaching the external border ; the point of bifurcation being in 

 general marked by the development of a more or less prominent 



Fig. 793. — Cceloceras {Ammonites) 

 annulatum. Lias. 



Fig. 794. — Cozloceras (Ammonites) 

 commune. Lias. 



tubercle. The species of the genus are confined to the Jurassic 

 rocks. 



Very closely allied to the preceding is the genus Cceloceras, but 

 the types included under this name have a long body-chamber (ex- 

 tending over more than one whorl), while the aperture of the shell 



