AMMONOIDEA. 



869 



the turns of which lie in one plane, but the volutions are not in 

 contact with one another. The surface-sculpture consists of trans- 

 verse ribs, usually with rows of tubercles or short spines. The 

 suture is much ramified, but only consists of four principal lobes, 

 viz., a siphonal lobe, superior-lateral lobe, inferior-lateral lobe, and 





Fig. 801. — Crioceras Emerici. 

 Cretaceous. 



Fig. 802. — Heteroceras Emerici. 

 Cretaceous. 



antisiphonal lobe. The body-chamber is long, and the aperture is 

 simple and without side-lappets. The species of Crioceras are Cre- 

 taceous, ranging from the Lower Greensand to the Gault. Some 

 of the forms which have been placed under the genus Heteroceras 

 appear to be nearly related to Crioceras. In these forms the first 

 portion of the shell resembles Turrilites in being obliquely coiled 

 and turreted, with contiguous whorls ; but the last volution is de- 

 tached from the rest, and is produced and recurved (fig. 802). 



The forms for w T hich the name of Toxoceras is employed have an 

 elongated, arcuate or bow-shaped shell, which is not coiled into a 



Fig. 803. — Ancy loceras Mather onianum. Gault. 



spiral ; and the structural characters agree with those of Crioceras. 

 The species of Toxoceras are found in the Lower Cretaceous rocks ; 

 but it is not clear that the genus has not been founded upon broken 

 examples of Crioceras. 



