868 



TETRABRANCHIATE CEPHALOPODS. 



ribs, which increase in width in proceeding from the umbilicus to- 

 wards the external margin, and are often adorned with tubercles. 

 In the genus Aspidoceras^ again, the shell (fig. 799) may be flat and 

 widely umbilicated ; or it may be inflated and involute, with a narrow 

 umbilicus. The sculpture consists of one or two rows of tubercles, 

 which occasionally become obsolete in late life, while ribs are as a 

 rule only present in young examples. The numerous species of 



Fig. 798. — Acanthoceras Deverzanitm. 

 Cretaceous (Lower Chalk). 



Fig. 799. —Asfiidoceras longisfiinum. 

 Jurassic (Kimeridge Clay). 



Acanthoceras are wholly Cretaceous, while Aspidoceras ranges from 

 the Middle Jurassic to the Lower Cretaceous. Allied genera are 

 Simoceras and Peltoceras, both of which are confined to the Jurassic 

 series. 



All the preceding forms of the Stephanoceratidce possess a spirally- 

 coiled shell, the volutions of which are contiguous and lie in a single 

 plane ; and all belong to the great group of the " Ammonites." 

 There are, however, other types, with a vari- 

 ously coiled shell, which, as judged by their 

 general morphological characters, appear to be 

 properly referable here. Amongst the types 

 in question is the genus Scaphites, as re- 

 stricted by the removal to the Lytoceratidce 

 of the forms which constitute the genus Ma- 

 croscaphites. In Scaphites proper the shell (fig. 

 800) consists of a series of volutions coiled 

 into a flat spiral, but having the last whorl 

 detached from the others, produced, and ulti- 

 mately bent back in the form of a crosier. 

 The suture-line shows several auxiliary lobes ; 

 and an " Aptychus " is present. All the known species of Scaphites 

 are confined to the Cretaceous system. 



In the genus Crioceras (fig. 801) the shell is rolled into a spiral, 



-Scafih ites cpqua- 

 Upper Cre 



retaceous 



