JURASSIC PERIOD. 



4J9 



4. MollUSks. — (a.) Conchifers.—Fig. 723, Ostrea Marshii, characteristic of 

 the Oxford (Kelloway) Clay; fig. 724, Exogyra Virgula, from the Upper Oolite 



Figs. 723-728. 



Conchifers.— Fig. 723, Ostrea Marshii; 724, Exogyra Virgula; 725, Gryphsea dilatata; 726, 

 Trigonia clavellata; 727, Astarte minima; 728, Diceras arietiua. 



(Kimmeridge Clay); fig. 725, Gryphsea dilatata; 

 fig. 726, Trigonia clavellata, Upper Oolite; fig. 

 727, Astarte minima, Coral limestone; fig. 728, 

 Diceras arietina, Coral limestone, a bivalve whose 

 valves are each elongated into the shape of a 

 curved horn, — whence the name, from Sis, twice, 

 and xepas, horn. 



(b.) Gasteropoda,. — Fig. 729, Nerinsea Goodhallii, 

 Coral limestone. The genus is remarkable for 

 the elongated form of the shells, and for having 

 one or more prominent ribs on the inner surface, 

 partly filling or contracting the interior. The 

 appearance of the aperture in one species is 

 shown in fig. 785 : the species are confined to the 

 Jurassic and Cretaceous periods. Purpuroidea 

 nodulata, from Middle Oolite, Pleurotomaria 



Fig. 729. 



Nerinsea Goodhallii. 



