JURASSIC PERIOD. 



439 



The Gasteropods were represented by several new modern genera, 

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



Fig. 7 



Figs. 787, 788. 



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



788, A. Bucklandi. 



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 Humphreysianus ; 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 



