R, Lydekker and Q. A. Boulenger — Notes on Che'oma. 273 



A crushed and broken shell with several of the bones of the 

 pectoral and pelvic girdles, obtained by the late Mr. Fox from the 

 Wealden of Brook, in the Isle of Wight, and now preserved in the 

 Museum (No. R. 171), together with another crushed specimen (No. 

 48349) from the Wealden, and the anterior portion of a plastron 

 (No. 46325) from the Purbeck, belong to the genus Tretosternum, 

 and enable us to arrive at a conclusion as to the affinities of that 

 genus. These specimens show, moreover, that the Chelonian remains 

 from the Wealden of Bernissart in Belgium, described by M. Dollo,^ 

 under the name of PeJtochelys Dachastelii, are not separable from the 

 English Tretosternum BaJcewelU (Mantell ^ sp.). M. Dollo was led to 

 separate the Belgian form because Sir R. Owen^ has suggested that 

 Tretosternum may have had incomplete marginals ; which the present 

 specimens show is not the case. The suggestion of Sir R. Owen* 

 that Tretosternum had a permanent plastral vacuity is shown by 

 these specimens to be incorrect, although a small one exists in the 

 young. M. Dollo places the genus in the Pleurodira, a view which 

 is not supported by the examination of the remains in the Museum. 



As may be seen from M. Dollo's figure, and still better from 

 the Museum specimens, the anterior border of the nuchal bone 

 is broadly emarginate, a character which occurs in none of the 

 Pleurodira, for the reason that the head takes shelter on the side ; 

 but is found in all Cryptodira in which the head is particularly large 

 and non-retractile, with the temporal fossEe roofed over by bone, as 

 in Chelydra, Platy sternum, Chelone, etc. Another peculiarity shown 

 by the nuchal befoi'e us lies in the presence of a costiform process 

 on each side, as in Chelydra ; a process which, judging from Dollo's 

 figures, is also shown in the young specimens from Bernissart. 

 Important again is the fact, shown by our specimens, that the plastron 

 articulates with the carapace by gomphosis, exactly as in Chelydra, 

 and does not send off any axillary and inguinal peduncles like those 

 so highly developed in all known Pleurodira. The plastron is essen- 

 tially of the Dacfylosternine-tjTpG of Cope. M. Dollo has, it is true, 

 recognized the presence of an intergular shield, a character which has 

 hitherto been regarded as infallibly diagnostic of the Pleurodiran 

 gi'oup; this shield is also well shown in the type-specimen of Treto- 

 sternum. A careful examination shows, however, that this intergular 

 is, in addition to only five pairs of plastral shields, exactly as in 



1 Bull. Mus. R. Hist. ISTat. Bel^. vol. iii. p. 78 (1884). The name itself is not 

 very happily selected, since Prof. Seeley (Quart. Journ. Geol. See. vol, xxxvi. p. 412, 

 1880), has proposed the name Peltocheliiidce for an entirely different group of Chelonia. 



'^ Geology of S.E. of England, p. 255 (1833) — Trionyx. Tretosternum punctatum, 

 Owen, dates from 1842. 



3 Eep. Brit. Assoc, for 1811, pp. 165-7 (1842). Sir R. Owen's words are, "The 

 entire and rounded margins of the truncated and expanded extremities of the ribs, 

 beyond which there is not the slightest trace of projecting tooth- like processes, 

 strongly indicates that the marginal plates [bones] were either wanting or rudimental, 

 as in the genus Cr>/ptopiis [Trioiri/x'].^^ 



* Op. cit. p. 167. The generic name was given upon the assumed existence of this 

 feature ; since, however, the vacuity occurs in the yoimg, no reasons can be adduced 

 for changing the name. 



DECADE III. VOL. IV. NO. TI. 18 



