ROOM III. TRETOSTERNUM BAKEWELLI. 157 



in Tilgate grit, is placed with the Wealden Chelonian remains 

 in this case. 



LIGX. 34 COSTAL PLATE OF A FRESHWATER TURTLE. TILGATE FOREST. 



(TRETOSTERNUM BAKEWELLI.) 



( nat. size.) 



TRETOSTERNUM (Trionyx Bakewelli. 1 } The most numerous, 

 and at the same time the most enigmatical remains of Chelo- 

 nians, obtained in my earliest, researches in the Wealden of 

 Sussex, were fragments of very flat costal plates of the cara- 

 pace of a species of freshwater turtle, having a granulated 

 external surface, and resembling in this respect the bony 

 case of the recent fluviatile sub-genus, named Trionyx, from 

 their having but three free toes on each foot. These Che- 

 lonians are distinguished by the intercostal spaces, the ribs 

 not being ossified throughout, and their extremities having 

 no osseous border of support; the external surface of the 

 carapace is covered with delicate pits or hollows, for the 

 attachment of the soft skin, which is the only dermal integu- 

 ment ; for the animals of this genus are destitute of scutes, 

 and consequently the dorsal and costal plates of the carapace 

 do not exhibit furrows or grooves, produced by the margins 

 of the scales of tortoise-shell, as in the other sub-genera. 



Such are the characters of the recent turtles, whose cara- 



1 Notice of the occurrence of this Turtle in the Wealden was first 

 given in my " Geol. of Sussex," 1822; it was figured and described 

 in "Foss. Tilgate Forest," in 1827; and again, as T. Bakewelli, in 

 " Geol. S. E. of England," 1833. 



