62 MR. R. LYDKKKER ON THE [Feb. J 9, 



replace it by Erquelinnesia. A year later (1887), M. Dollo ', 

 having had his attention directed to the circumstance that the name 

 Glossochelys had been previously applied to one of the forms which 

 were included in his Erquelinnesia, and also to the American types 

 described by Prof. Cope, came to the conclusion that Euclastes, 

 Lytoloma, ?ome of the forms included in Puppiyerus, Glossochelys, 

 and Ei-quelinnesia, all belong to one arid the same genus. It was at 

 the same time considered tiiat the earlier name Osteopygis might 

 also indicate the same form, but since the skull wns unknown its 

 adoption seemed inadvisable ; and it was accordingly proposed that 

 the term Euclastes, as being the earliest of those based on the 

 evidence of the skull, should be the one to be employed in this sense. 

 Unfortunately, however, this arrangement could not be accepted, 

 since, as the present writer has pointed out in a communication 

 recently made to the Geological Society, the name Euclastes is pre- 

 occupied. Accordingly, in that communication it was suggested, 

 assuming M. Dollo to be correct in his identification of Lytoloma 

 ^\\.h. Erquelinnesia = Glossochelys, that the forme; name, as being 

 the second earliest of those based on parts of the skull, should be 

 adopted. 



In the same communication it was also pointed out that the so- 

 called Chelone longiceps, which it seemed incumbent to take as the 

 type of the genus Pupjiigerus, was closely allied to the Bracklesham 

 Middle Eocene species originally described as Chelone trigoniceps, 

 and that, although the latter differed somewhat in the form of the 

 mandibular symphysis from typical forms of Lytoloma, yet these 

 two species must be classed in the latter genus, as had been proposed 

 by M. Dollo, at the time he employed the name Pachyrhynchus in 

 the same sense. 



Having now cleared up this intricate web of synonymy, attention 

 may be directed to the features in which Lytoloma differs from 

 existing Chelonidce, and the opinions which have been held as to its 

 affinities. 



In describing the Chelonidte of the London Clay, Sir Richard 

 Owen included in the lenu Chelone not only the Edible and the 

 Hawksbill Turtles, but also the Loggerhead, which is now generally 

 regarded as entitled to generic distinction, and forms the type of the 

 genus Thalassochelys, that genus typically differing from Chelone 

 in the absence of ridges on the palate and mandibular symphysis", in 

 the greater relative length of the latter, the lower alveolar walls of the 

 palate and symphysis, and in the tendency to an earlier obliteration 

 of the vacuities in the plastron, as well as in certain other skeletal 

 features which need not be mentioned here. It should be observed, 

 liowever, that all the features in which this genus differs from 

 Chelone are those of less specialization. 



In his original description Sir Richard Owen pointed out that the 

 specimen under consideration was closely allied in structure to the 

 skull of the so-called Chelone jilanitnentum. And it was shown that 



1 HjIcI. vol. V. p. 261 (1888), and Bull. Soc. Gtol. Nord, vol. xr. p. 114 (1889). 

 ^ These ridges are present in the Mexican Loggerhead. 



