CHELONIA. 45 



Family — Fluvialia. 

 Genus — Tr i o n yx . 



The Chelonian Reptiles called " Soft Tortoises," forming the genus Triojiyx of 

 Geoffrey St. Hilaire,* and the family Fliivialla sen Potamites of MM. Dumeril and 

 Bibron,f resemble those of the genus Chelone (family Marina seu Thalassiten, Dum. 

 and Bibr.) in the extremity of the vertebral rib, or pleurapophysis, projecting freely 

 from below the end of the connate costal plate,! ^^^ ifi having the plastron incom- 

 pletely ossified ; but they are characterised by the still more incomplete ossification of 

 the margin of the carapace, which retains much of its primitive soft, cartilaginous state ; 

 and they are further distinguished by the reduced number of the toes — three on each 

 foot — which are armed with claws, the other two toes serving to support a swimming 

 web ; the name of the genus has reference to this peculiarity. 



The head is depressed, elongated, and, in the recent animal, the nostrils are pro- 

 longed into a short tube, terminated by a small fleshy appendage like an elephant's 

 proboscis. The outer surface of the dermal bones of the carapace, and of the cor- 

 responding parts of the plastron^ is variously sculptured, usually by sinuous grooves 

 and rugosities, as if wormeaten ; and to such a degree in some species, as to give the 

 parts a tuberculate character. The cuticle is soft and flexible, not developed into 

 scutes ; and there are accordingly no impressions like those that indicate the presence 

 of the " tortoise-shell" plates in the skeleton of the existing turtles and in the petrified 

 plastrons and carapaces of the extinct species of the marine family. 



" Hitherto," write the meritorious authors of the elaborate ' Erpctelogie Generale,' 

 one has not observed any species of this family {Potamites) in our European rivers ; 

 all those which have been described, and of which the habitat is known, have come from 

 the streams, rivers, or great fresh-water lakes of the warmer regions of the globe." (Tom. 

 ii, p. 469.) The beautifully-preserved evidences of the species about to be described, 

 which have chiefly been obtained by the Marchioness of Hastings from one limited 

 locality, attest the abundance of the Trionyces in the fresh-waters of our latitudes 

 during the Eocene period of geology. 



The characters by which MM. Wagler and Dumeril have divided the species of 



* Annales du Museum d'Histoire Naturelle, torn. xiv. 

 t Erpetologie Geuerale, 8vo, torn, ii, p. 4C1. 



X This cliaiacitcr is well exemplified in the Marchioness of Hastings's unique and beautiful specimen of 

 the Trionyx rivosus, T. XVIII^. 



