JURASSIC PERIOD. 



489 



The Gasteropods were represented by several new modern genera, 

 besides others that are now extinct. One of the more peculiar forms 



Fig. 786. 



Figs. 787, 788. 





Gasteropod. — Fig. 786, Nerinaea Goodhallii. Cephalopods. — Fig. 787, Ammonites spinatus ; 



788, A. BueklandL 



was that of the genus Nerincea (Fig. 786), in which the spiral cavity- 

 has one or more ridges, as shown in Fig. b. 



But the type of Cephalopods especially underwent great expansion. 



Figs. 789, 790. 



Cephalopods. —Fig. 789, Ammonites Humphrevsianus ; 790, A. Jason. 



The group of " Ammonites abounded in species. Figs. 787, 788, are 

 Liassic species ; and Figs. 789, 790, others from the Oolyte. The last 

 two figures have the aperture unbroken ; and in 790 it is much pro- 

 longed on either side. 



In addition to these Cephalopods with external chambered shells 

 (Tetrabranchs or Tentaculifers), there were also those having an in- 

 ternal shell or bone (Dibranchs or Acetabulifers), a group which 



