406 CHELONIA 



CHAP. 



nuchal plate has usually a pair of rib-like processes. The ueurals 

 form a continuous series, except in the African Cydanorhis, in 

 which they are much reduced in size, and separated by the costal 

 plates. 



The plastron is imperfect, all its constituent nine elements 

 being only loosely connected with each other, and there remains 

 a wide median vacuity between the Literal elements. Most of 

 these plastral bones are reduced to splints, which, instead of 

 meeting by regular sutures, loosely interdigitate with their 

 jagged edges. In the young all these ventral elements are deeply 

 imbedded in the soft, leathery skin, and 'they do not at all 

 resemble in appearance those of the dorsal side. With age they 

 develop upon their ventral surface stronger and denser ossifica- 

 tions, which ultimately broaden out, sometimes beyond the 

 original underlying bone, and assume the characteristic vermi- 

 culated surface -appearance. This is undoul)tedly a process of 

 exostosis, a step towards revival of that armour wliich had been 

 much reduced ancestrally. To appreciate this condition, it is at 

 least suggestive that these mud-tortoises, when kept in the usual 

 hard-bottomed tanks, invariably become sore, the skin wearing 

 through where the imbedded plastral bones touch the ground. 

 Thus what is crammed into the short life of a captive individual, 

 is in the natural course of events spread over many generations, 

 wliereby it has ceased to be pathological, and has become a com- 

 paratively new, tertiary, but regular feature. 



It is not open to much doubt that the characteristic features 

 of the Trionychoidea are not primitive but secondary. This is 

 indicated by the w^hole structure and behaviour of the carapace 

 and plastron. The softening of the whole shell, the loss of the 

 horny shields, the reduction of the claws, are the direct and 

 almost unavoidable results of life in muddy waters. 



Geologically they do not seem to be very old. They appear, 

 already referable to the genus Trionyx, in the Upper Cretaceous 

 strata of North America. In the Lower and Middle Tertiary 

 strata many species existed in North America and in Europe, 

 and it is of great importance that in these species the costal 

 plates were much broader, and the marginal plates better developed, 

 than ill the recent forms. Now their lialf- dozen genera, 

 with about twenty-four species, are confined to North America, 

 the tropical and warmer parts of Asia, and the Malay 



