46 FOSSIL REPTILIA OF THE LONDON CLAY. 



Trionyx, Geoffr., into two genera, are not such as can be decisively recognised in the 

 fossil carapace. A difference of convexity of that part by which the " Cryptqpodes" 

 are said to differ from the Gymnqpodes, is not one that the comparative anatomist and 

 palaeontologist would recognise as valid for the distinction proposed. 



Upon the whole, the fossil specimens in which that character can be compared, 

 agree rather with the Gymnqpodes of Dum. and Bibr., but with a range of diversity which 

 is exemplified by Tab. XVI^, and XVII. So much of the plastron as I have been 

 able to compare, agrees likewise with the bones of that part in the Gymnopodes, but, 

 in the absence of more certain characters, and with doubts as to the necessity or 

 desirableness of the subdivision proposed for the recent species, I shall retain the 

 name Trionyx for all the fossils that manifest, in their petrified remains, the characters 

 of the Geoffroyan genus. 



In the second part of my 'Report on British Fossil Reptiles' I showed that certain 

 fossils of the Weal den formation and of the Caithness slate (new red sandstone) had 

 been referred erroneously to the genus Trionyx, and that the only unequivocal remains 

 of that genus which had been seen by me at that period (1841) were from Eocene 

 deposits at Sheppy, Bracklesham, and the Isle of Wight, in which latter locality they 

 were associated, as in the Paris basin, with remains of the Anqplotherium and 

 PalcBotherium. 



I have since had the opportunity of examining fossil specimens of Trionyx from 

 other localities, but always, however, from formations of the Eocene period, and I 

 shall commence their description with one of the most perfect and beautiful examples 

 of these Chelonites, which was obtained by the Marchioness of Hastings from the 

 Eocene sand of the Hordwell Cliff, Hants. 



Trionyx Henrici. Owen. Tab. XVI. 



Report of the Seventeenth Meeting of the British Association, 1S47, p. 65. 



Although the characteristics of the genus are readily recognisable in fossil 

 fragments of the carapace and plastron, from their comparative flatness and the 

 sculpturing of the outer surface, the species of Trionyx are with difficulty determinable, 

 if at all, from such specimens ; and it is usually necessary to have a considerable part 

 of the carapace, in order to ascertain its composition, contour, and degree of convexity. 

 Some species, indeed, e. g. Trionyx rivosus (T. XVIIL4), Trionyx maryinatus (T. XIX*), 

 together with the Trionyx spinosus* and Trionyx sidcahis of Kutorga, would seem to be 

 characterised by particular patterns of the irregular surface of the bones of the 

 carapace, which character, therefore, a fragment may suffice to manifest ; but this 

 is not the case with the ordinal*}' rugose and vermiculate species. Cuvier accordingly 



* This is quite a distinct species froni the Trionyx spiniferus of Lesueur. 



