274 R. Lydehker and O. A. Boulenger — Notes on Chelonia. 



certain specimens of Dermatemys, instead of the six pairs of all 

 recent Pleurodira ; and we think too great importance has hitherto 

 been attached to the disposition of the epidermic plastral shields, as is 

 shown by a specimen of Macroclemmys Temminclcii in the Museum, 

 which exhibits an intergular shield situated within the margin of 

 the plastron, exactly as in the Australian Chelodina. Again, the 

 number of plastral shields of Tretostermim, Yiz. 11, is that of a 

 Cryptodiran, and not of a Pleurodiran. In addition to the above 

 arguments, the remains before us give elucidation on a point which 

 is in itself decisive ; viz, the attachment of the pelvis. Two bones 

 only are preserved, but they will suffice for our purpose ; these are 

 a sacral vertebra and a pubis. The former bone shows a large 

 articular surface for a sacral rib, which was therefore present, and 

 gave attachment to the ilium. In the Pleurodira, in which the ilium 

 anchyloses with the carapace, sacral ribs are either atrophied or 

 disappear altogether in the adult. The pubis, of a rather peculiar 

 shape, owing to the great elongation and distal expansion of the 

 inner or symphysial branch, is intact and shows that the extremity 

 of the outer process was thin and compressed, and did not anchylose 

 with the plastron.' 



In the absence of any remains of the caudal vertebral column, it is 

 impossible to say for certain that Tretosternum belongs to the family 

 CJielydridcB, but we may surmise that such was probably the case. 

 As far as can be judged from a comparison with Leidy's figure of 

 Anostira, the two genera were very closely allied. 



The last of the Chelonians from the Wealden and Purbeck that 

 we have to notice is a small cordiform carapace from the latter 

 formation (B. M. No. 40676), which may confidently be referred to 

 the Thallasemydes of Prof. Riitimeyer {EurysternidcB of Dollo), and 

 may probably be referred to the genus Eurysterniim.^ 



London Clay. — The first specimen from this formation that calls 

 for notice is the hinder portion of a carapace in the Museum (No. 

 40099), which evidently belongs to the genus Pseudotrionyx, Dollo,^ 

 from the Middle Eocene (Bruxellian) of Belgium, and is apparently 

 specifically identical with the typical P. Delheidi, Dollo. This genus 

 is included by its founder in the Chelydridcs, but from the absence of 

 epidermal shields may probably be regarded as belonging to a distinct 

 family.* 



The next species for notice is founded on the shell figured by 

 Owen and Bell ^ under the name of Platemys Bowerbanhi ; ^ in regard 



^ On this occasion it may be well to observe that Eiititneyer's statement, made 

 "with due reserve, but since repeated by others as an admitted fact, that, in some 

 recent Pleurodira, the ischium anchyloses with the plastron before the pubis, was 

 derived from the examination of figures accompanying one of Gray's memoirs ; and 

 that an investigation of the actual specimens which gave rise to this statement has 

 proved it to be erroneous.— [G. A. B.] 



2 = Acichelys = Aplax = Falmomedusa = Acheloiiia — Euryaspis = Farachelys. 



3 Bull. Mus. E. Hist. Nat. Belg. vol. iv. p. 96 (1886). 



* The suggestion made in the above-quoted notice in the Geological Magazine 

 that the genus might perhaps be referred to the Trionyehidce will not hold. — [R. L.] 

 ^ Reptilia of the London Clay, pt. i. pi. xxiii. 

 6 Eep. Brit. Assoc, for 1841, pp. 163 (1842). 



