CHELONID^. 53 



have been included) was originally proposed, cannot be regarded as 

 of even generic value, since similar variations occur in Trionyoc l . 

 Again, the difference in the contour of the humerus of the adult 

 of the larger species from that of Chelone, on which Dollo in his 

 later memoirs mainly relies in his retention of the Propleuridce, 

 cannot be regarded as more than a generic character, since there is 

 an extremely close connection between this type of humerus and 

 that of Thalassochelys ; some of the species included in the present 

 genus having, indeed, the head of this bone placed even less obliquely 

 than in the existing genus. The alleged separation of the nasals 

 from the prefrontals is noticed below. 



Finally, seeing that the existing Thalassochelys caretta has an 

 almost cosmopolitan range, it has yet to be proved that the N.- 

 American forms described as Eudastes and Lytoloma are even specifi- 

 cally distinct from the European ones. 



Lytoloma trigoniceps (Owen 2 ). 



Syn. Chelone trigoniceps, Owen 3 . 

 Chelone acuticeps, Owen 4 . 

 Pachyrhynchus trigoniceps, Dollo 5 . 



Apparently allied to L. longiceps, from which it is distinguished by 

 the wider and more concave interorbital space, the longer and more 

 depressed orbits, in which the frontal border is entire, the less emar- 

 ginate pterygoids, and the narrower inferior apertures of the temporal 

 fossae. In both species the interparietal shield is long and narrowed 

 in front. In the present species the inferior aspect of the mandibular 

 symphysis may be somewhat convex, with an indistinct longitudinal 

 keel, but in other cases it is nearly flat. The neurals of the carapace 

 are similar to those of L. longiceps. This species attains larger 

 dimensions than any examples of the latter species. 



In some of the foregoing characters this species approximates 

 to Thalassochelys, and thus tends to connect the larger Lower Eocene 

 species with that genus. If any of the following specimens are 

 adult, the species must have been of comparatively small size. 



The variations in the degree of flattening of the mandibular 

 symphysis are probably sexual. 



Hob. Europe (England). 



1 See Boulenger, ' Catalogue of Chelonians, &c.' pp. 244-245. 



2 In Dixon's 'Geology of Sussex,' 1st ed. p. 218 (1850).— Chelone.- Also 

 mentioned in 'Fossil Eeptilia of London Clay' (Mon. Pal. Soc), vol. i. pt. i. 

 p. 31 (1849), without figure. 



3 Loc. cit. 



4 History of British Fossil Eeptilia, Chelonia, pi. xxv. (1849). 



5 Bull. Mus. E. Hist. Nat. Belg. vol. iv. p. 138 (1886). 



