CflELYDEID^. 137 



ments of some of the bones themselves ; from Rott. This 

 specimen is considerably larger than the preceding, and 

 agrees very nearly in this respect with the type skeleton 

 from the same beds, figured by Meyer in the ' Palseonto- 

 graphica,' vol. ii. pi. xxviii. The middle of the nuchal 

 bone is preserved, and the contour of the anterior ver- 

 tebral and costal shields is clearly indicated. 



Purchased, 1855. 



Subfamily Tretosterninj. 



Carapace with pustulate sculpture and epidermal shields ; no per- 

 sistent vacuity in the plastron, which has a long bridge; a straight 

 suture between the hypo- and xiphiplastrals ; neural bones hexa- 

 gonal, long and comparatively narrow, with short antero-lateral 

 surfaces, number unknown ; caudal vertebrae and number of mar- 

 ginals unknown. Provisionally included in the family. Baur 1 

 states that Toecochelys, Cope, of the American Cretaceous, which he 

 regards as a true Chelydroid, has proccelous caudal vertebras and 

 the tympanic ring open. 



Genus TRETO STERNUM, Owen 2 . 



Syn. Peltochelys, Dollo 3 . 



The type and only definitely known genus of the subfamily. 

 Plastron with an intergular and five pairs of plastral shields. An- 

 terior border of carapace deeply emarginate, and the skull probably 

 therefore very large ; dorsal surface of neurals sculptured. The 

 plastron has distinct axillary and inguinal notches, and the xiphi- 

 plastrals are not notched. Skull known only in young. It is 

 somewhat difficult to know whether to regard the azygous median 

 plastral shield as coalesced gulars or as an intergular ; if the former 

 view be adopted, the shields here termed gular will be humeral. 



Whether the imperfect Chelonian remains from the Cretaceous of 

 the United States, described by Leidy 4 under the name of Comps- 

 emys, are entitled to generic distinction from Tretosternum remains 

 to be proved. The type specimens are described as being covered 

 with granular tubercles. Peltochelys is noticed below. 



1 Ann. Mag. Nat. Hist. ser. 6, vol. iii. p. 58 (1889). 



2 Rep. Brit. Assoc, for 1841, p. 165 (1842).— Tret osternon. 



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



4 Proc. Ac. Nat. Sci. Philad. 1856, p. 312. See also Cope, Rep. U.S. Geol. 

 Surv. Terrs, vol. ii. pi. vi. figs. 15, 16 (1875). 



