MESOZOIC FAUNAS 187 



Among Pelecypoda the Prionodesmacea maintained, 

 and even increased, their profusion, while many types 

 of Anomalodesmacea and Teleodesmacea came into 

 prominence. Although Aspidobranchiate Gastropods 

 showed a decline, it was by no means precipitous, and 

 rapid differentiation of Ctenobranchiate forms more 

 than compensated for the reduction. But it is in the 

 overwhelming outburst of specialization in the Cephalo- 

 poda that the Molluscan wealth of the Mesozoic mainly 

 resides. The Nautiloids were reduced to few types ; but 

 the Ammonoids reached their acme in the Trias, retaining 

 numerical and differential preponderance throughout the 

 Jurassic ; and Belemnoids, though monotonous in form, 

 must have swarmed in most Mesozoic seas. 



Palaeoconch Pelecypoda were feebly represented in 

 the Mesozoic by one family only (Solenopsidae) which 

 is still living. Taxodonts are locally abundant. Leda 

 and Nucula are not uncommon in the Lias, and the 

 latter genus, accompanied by its near ally Acila, is 

 represented by abundant and characteristic species in 

 the Gault. Cucullaea is often found in the Lower Oolites 

 and Upper Greensand, and early forms of Limopsidae 

 and Arcidae occur in the Jurassic and Cretaceous. 

 Schizodonts were developed to an extraordinary degree. 

 The most important families were the Pernidae, Pteriidae, 

 Ostreidae, Unionidae and Trigoniidae. Gervillia^ a 

 soleniform member of the first family, is common in 

 shallow water deposits from the Trias to the Lower 

 Cretaceous ; Perna (Pedalion) is known from the 

 Inferior Oolite, but is specially characteristic of the 

 Lower Greensand ; while Inoceramus and its allies, 

 commencing in the Lias, are perhaps the most abundant 

 and universal fossils of the Cretaceous period. Truly 

 gigantic species occur in the Upper Chalk ; prismatic 

 fragments can be found in almost every quarry at any 



