86o 



TETRABRANCHIATE CEPHALOPODS. 



Fig. 778. — Lytoceras jimbri- 

 atum. Lias. 



and saddles being divided into small symmetrical digitations. The 

 species of Lytoceras range from the base of the Lias to the middle 

 of the Cretaceous system. 



The genus Macros caphites (fig. 779) 

 comprises a portion of the forms included 

 in the old genus Scaphites, and the suture- 

 lines agree with those of Lytoceras and its 

 allies in the fact that the lateral lobes are 

 divided into two symmetrical halves. The 

 first portion of the shell is coiled into a 

 flat spiral, the volutions of which are in 

 contact, but the last-formed portion of the 

 shell is disjunct and is prolonged in a 

 straight line, its terminal extremity being 

 bent upwards and backwards. The species 

 of this genus are confined to the Lower 

 Cretaceous rocks. 

 In the genus Hamites (fig. 780) the shell is an extremely elon- 

 gated cone, which is bent upon itself more than once in a hook- 

 like manner, all the volutions being separate. 

 The surface is usually adorned with trans- 

 verse ribs, and the body-chamber is very 

 long. The suture resembles that of the 

 Lytoceratidce in the fact that the superior- 

 lateral lobe (as also commonly the inferior- 

 lateral lobe) is divided into two symmetrical 

 halves, the saddles also being bilaterally 

 symmetrical. The species of Hamites range 

 from the Neocomian to the Chalk, and 

 some forms attain a considerable size. 

 The Cretaceous (Neocomian) genus Hamu- 

 lina differs from Hamites in the fact that 

 the elongated shell is only bent once, its 

 two portions being parallel to one another 

 but not in contact. In the genus Ptycho- 

 ceras, again, the shell (fig. 781) has the 

 form of a greatly elongated cone, which is 

 once bent upon itself, the two straight por- 

 tions of the shell being in contact. The 

 thicker limb of the shell is transversely 

 ribbed ; and the sutures have the same 

 form as those of Hamites. The species of 

 Ptychoceras are exclusively Cretaceous, and range from the Neo- 

 comian to the Gault. 



In the genus Turrilites (fig. 782) the shell has the form of a 



Fig. 779. — Macroscaphites 

 Ivanii. Cretaceous. (Neo- 

 comian.) 



