CHELYDRTDJE. 141 



Tretosternum punctatum, Owen 1 . 



The type species. Of considerable size, with the sculpture on 

 the plastron in the form of distinct conical pustules, and the outer 

 extremities of the costal bones not greatly enlarged. 



The type specimens are from the Purbeck, and were in the Col- 

 lection of Sir P. Egerton ; but nothing is now known of their 

 whereabouts. 



Hob. Europe (England) . 



R. 171. An imperfect carapace and plastron, in a number of frag- 

 ments ; from the Wealden of the Isle of Wight. Noticed 

 by Boulenger and the present writer in the ' Geol. Mag.' 

 dec. iii. vol. iv. p. 273. The chief fragments are indi- 

 cated by separate letters, as follows : — a. The nuchal bone, 

 imperfect posteriorly : this specimen shows the deep an- 

 terior emargination, together with the base of the costi- 

 form processes characteristic of the Chelydridce. b. Three 

 neural bones, with portions of the arches of the subjacent 

 vertebras ; the vertebral centra were evidently far below 

 the level of the carapace, with a vacuity above the cen- 

 trum of each segment, as in existing Chelydridce. c. A 

 neural attached by its dorsal surface to the ventral aspect 

 of the imperfect eighth costal of the left side ; the neural 

 is elongated, d. Part of the supra-pygal region of the 

 carapace, with a sacral vertebra attached ; the vertebra, 

 as noticed on p. 274 of the memoir cited, shows a large 

 attachment for a sacral rib, which is decisive as to the 

 Cryptodiran affinities of the genus 2 . d'. Six imperfect 

 costal bones, d". The imperfect eighth costal of the right 

 side. e. A number of marginal bones and other portions 

 of the peripheral region of the carapace. /. The im- 

 perfect plastron, comprising portions of the epiplastrals, 

 the entire entoplastral, the two hyoplastrals, and the left 

 hypoplastral. The right hyoplastral consists of two por- 

 tions, the lower of which would seem at first sight to 

 represent a mesoplastral element; but since no similar 

 division is observable on the opposite side, it would seem 

 that this line of division is accidental. 



Fox Collection. Purchased, 1882. 



1 Eep. Brit. Assoc, for 1841, p. 165 (1842). 



2 In Pleurodira the ilium anchyloses to the carapace, and in the adult sacral 

 ribs are either atrophied or totally wanting. 



