62 BRITISH FOSSIL REPTILES. 



toes, as ixi Tetroni/x, Lesson, and usually all five, are armed with claws, and are united 

 together by a web only at their base ; but the extent of this web and the length and 

 flexibility of the digits vary in the dififerent species and sub-genera, and accordingly 

 they manifest various degrees of aptitude for swimming, or for climbing the banks of 

 the streams or marshes which they habitually frequent, and for walking on dry land. 



The costal plates extend, in the mature individuals, to the ends of the ribs, and 

 articulate with the marginal plates ; the dermal pieces of the plastron are co- 

 extensive with the abdominal integument, and unite together by suture so as to form 

 an unbroken expanse of bone ; the sides of which, formed by part of the hyosternals 

 and hyposternals, unite with a corresponding proportion of the lateral borders of the 

 carapace. There is a gradation in the degree of convexity of the carapace, and in the 

 angle at which the sides of the plastron bend up to join the carapace, which pro- 

 gressively brings the marsh tortoises nearer to the true land tortoises {Terrestria), and 

 some of the steps in this progression of affinities are illustrated by the fossils from 

 the London clay. 



Those that, by the flatness of their carapace and plastron, depart least from the 

 flmaatile forms of the order will be first described. 



Genus — Platemys. 

 Platemys Bullockii. Owen. Plate 4. 



Eeport ou British Fossil Reptiles, Trans. British Association, 1841, p. 164. 



Amongst the fossil Chelonians of the London clay, the portable dwelling-house of 

 which was provided with side walls as well as a floor and roof, are some tolerably 

 large species, remarkable for the lowness of the roof of their abode, and especially for 

 the flatness of its floor. 



A rigid comparison of the numerous species of the marsh-dwelling Chelonians, 

 which the active I'csearches of naturalists have brought within the domain of science, 

 has led to their classification into several groups, to which generic or sub-generic 

 names are attached, and the fine preservation of the characteristic part of the 

 skeleton of the specimen from Sheppy, figured in PI. 4, gives the opportunity for 

 determining to which of these subdivisions of the genus Emi/s of Bronguiart that 

 specimen belongs. 



In ray ' Report on British Fossil Reptiles,' the result of these comparisons, as 

 regards the present fossil, were simply indicated by the sub-generic name, and I 

 confined myself to a description of the specific distinctions noticeable in the only 

 e.xample I had then seen. 



The present species difi"ers from all those to which MM. Dumeril and Bibron 



