138 CHELONIA. 



The sculpture of the shell is the same as in the Neocomian genus 

 Hdochelys, which is noticed among the Amphichelydia, and is at 

 once distinguished by the presence of a mesoplastral element. 



Tretosternum bakewelli (Man tell 1 ). 



Syn. Trionyx bakeicelli, Mantell 2 . 



(?) Peltochelys duchastellii, Dollo 3 . 



Apparently smaller than the next species, with the sculpture on 

 the plastron more or less vermiculated instead of simply pustulate, 

 and the costal bones much expanded distally. 



The generic identity of Peltochelys with Tretosternum was indi- 

 cated in a paper published by G. A. Boulenger and the present 

 writer in the ' Geol. Mag.' dec. iii. vol. iv. pp. 273, 274 (1884). 

 All the specimens of Tretosternum were, however, then referred to 

 a single species ; the Belgian form is provisionally referred to the 

 present species as the commoner Wealden type. 



Hab. Europe (England and Belgium). 



All the known specimens are from the Wealden. 



2265. Slab showing the dorsal surface of a costal bone ; from the 

 {Fig.) Wealden of Cuckfield, Sussex. The type specimen ; figured 

 by Mantell in his ' Illustrations of the Geology of Sussex,' 

 pi. vi. fig. 1 (1827), without name, and also in his ' Geology 

 of the South-east of England,' p. 255, as the type of 

 Trionyx balcewelli. On p. 167 of the ' Rep. Brit. Assoc' 

 for 1841, these and the other specimens figured by the 

 same writer are identified by Owen with his Tretosternum 

 punctatum of the Purbeck. The specimen shows the outer 

 sulcus of one of the vertebral shields, and another sulcus 

 dividing two costal shields and joining part of a marginal 

 sulcus. The vertebral shields were evidently of the wide 

 type characteristic of existing Chelydridce. 



Mantell Collection. Purchased, 1838. 



3598. The anterior extremity of the plastron, imperfect on the left 

 {Fig.) side, and partly embedded in matrix; from Cuckfield. 

 This specimen, which very probably belongs to the same 

 individual as No. 2265, and may be regarded as one of the 

 types, is figured by Mantell in his ' Illustrations of the 

 Geology of Sussex,' pi. vi. fig. 3, and noticed by Boulenger 



1 Geology of the South-east of England, p. 255 (1833).— Trionyx. 



2 Loc. cit. 3 Bull. Mus. R. Hist. Nat. Belg. vol. iii. p. 78 (1884). 



