282 TEETIAET VEETEBEATA OE THE EATtM, 



of Bournoncle-St.-Pierre (Allier), differs from the present species in the form of the 

 plastron, the anterior lobe of which narrows gradually forwards to a point; it also 

 differs in the great convexity of the carapace, the other characters of which 

 unfortunately have not been described. 



Another Tertiary giant Tortoise, with which T. ammon may be compared, is 

 T. perpiniana, described by Deperet and Donnezan* from the Lower Pliocene of 

 Eoussillon. This species also attained a very large size, one shell measuring 

 120 cm. in length. The chief points in which it differs from T. ammon are : — 

 (1) the carapace is much less convex and is smooth ; (2) there is no nuchal ; (3) the 

 plastron, though somewhat similar posteriorly, differs anteriorly in narrowing more 

 rapidly and in not having the epiplastral prolongation (which, however, may be a 

 sexual character). 



T. leberonis, Deperet f, from the Miocene of Mt. Leb^ron, is very similar to 

 T. perpiniana, but still larger, the shell attaining a length of 150 cm. It has also 

 more slender epiplastra and xiphiplastra, and therefore approaches the ^Jresent species 

 in these points rather more nearly than does T. perpiniana. The gigantic extinct 

 Tortoises of India, such as T. atlas, seem to be closely similar in many ways to these 

 European Tertiary forms, particularly in the absence in most of them of the nuchal 

 shield — a character which distinguishes them from the present species. 



When the original description of this species was written the presence of a nuchal 

 shield had not been recognised, and consequently it was considered that it was more 

 nearly related to the European and Indian Tertiary forms than now appears to be 

 the case. So far as the evidence goes now, it seems that the nearest relationship is 

 with the Aldabra and Mascarene types and perhaps with some of the Indian forms, 

 in which a nuchal is also present. 



Several more or less nearly complete humeri of Land-Tortoises have been found 

 in the same beds as the shells, and though none of them are large enough to have 

 belonged to an animal of the size of the type specimens of T. ammon or T. headnelli, 

 nevertheless they may be regarded as probably referable to small specimens of one 

 of these species or of T. isis. The shaft forms a strong sigmoid curve. The ulnar 

 and radial crests are both strongly developed and make a very acute angle with one 

 another ; they are separated by a deep fossa : the ulnar crest rises considerably above 

 the head, which is an elongated oval in form. The dorsal surface of the upper 

 part of the shaft is marked by a strong ridge with a deep groove on its inner side 

 for the insertion of muscles. The middle of the shaft is comparatively slender and 

 is triangular in section. The distal expansion is not so great as in most of the large 

 Land-Tortoises, e. g. T. gigantea. The ectepicondyle is rather small ; there is a 

 long and deep ectepicondylar groove which perforates the bone at its lower end. 



* Comptes Eendus, vol. cv. (1887) p. 1275 ; also Mem. Soc. Geol. France, Mem. 3 (1890) p. 140. 

 t Comptes Eendus, vol. ex. (1890) p. 915. 



