380 CHELONIA 



marginals is 23, sometimes 25 individually. The plastron (Fig. 

 66, p. 321) is composed of the usual nine plates, which, however, 

 remain entirely free from the marginals, and are only loosely 

 connected with each other, enclosing a very large unossified 

 space. The horny shields covering the plastron number 13, and 

 there is a series of about 5 inframarginals (Fig. 61, 6, p. 315), 

 There are normally 12 pairs of marginal shields, a nuchal, 

 5 neural, and 5 or V costal shields. Whilst the number of 

 these dorsal shields is pretty constant in Clielone, it is subject to 

 an astonishing amount of individual variation in ThalassocJielys. 



The Chelonidae are a highly specialised offshoot of the 

 Cryptodira adapted to marine life. Fundamentally they agree 

 most with the Testudinidae, paradoxical as this may appear at 

 first sight. There is nothing primitive about them except the 

 complete series of inframarginal shields. Fossil forerunners of 

 marine t\irtle-like creatures appear in the Upper Jurassic deposits 

 of Europe and North America. The numerous genera have been 

 grouped together as Thalassemydidae and Chelonemydidae. They 

 are more or less intermediate between Chelonidae and Umys-like 

 Chelonidae, the carapace being not too much flattened and 

 broadened out, the fontanelles between the ribs are mostly small, 

 the plastral bones are still broad, enclose a smaller ossified space, 

 and there is still a bony bridge in most cases. The paddle-shape 

 of the limbs is less pronounced, and sometimes only indicated. 

 In some forms, especially Lytoloma, from the Upper Cretaceous and 

 Eocene of North America and Europe, the anterior portion of the 

 skull is much longer than in the Chelonidae, the vomerand the 

 premaxillaries are elongated, and the anterior portion of the roof 

 of the mouth, with the corresponding parts of the lower jaw, 

 seems to have carried crushing pads. Some of the best-known 

 Upper Jurassic genera are Eury sternum and Idiochelys; Plesioclielys 

 from the Purbeck and Wealden ; Allopleuron hofmanni from the 

 Upper Cretaceous of Belgium approaches Clielone by the large 

 fontanelles between the small marginal and the short costal 

 plates. True Chelonidae are very rare and imperfect in the 

 Mid-Tertiary strata, but both recent genera seem to have existed 

 since Pliocene times. 



The few recent Chelonidae are entirely marine, going on land 

 only in order to deposit their eggs in the sands of unfrequented 

 shores. Their distribution, in conformity with their oceanic life, 



