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Show 62 MR. R. LYDEKKER ON THE [Feb. ) 9, replace it by Erquelinnesia. A year later (1887), M. Dollo1, having had his attention directed to the circumstance that the name Glossochelys had been previously applied to one of the forms which were included iu his Erquelinnesia, and also to the American types described by Prof. Cope, came to the conclusion that Euclastes, Lytoloma, some of the forms included in Puppigerus, Glossochelys, and Erquelinnesia, all belong to one and the same genus. It was at the same time considered that the earlier name Osteopygis might also indicate the same form, but since the skull was 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 preoccupied. Accordingly, in that communication it was suggested, assuming M. Dollo to be correct in his identification of Lytoloma with Erquelinnesia **= Glossochelys, that the former 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 Puppigerus, 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 Chelonidee, and the opinions which have been held as to its affinities. In describing the Chelonidee of the London Clay, Sir Richard Owen included in the term 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 symphysis2, 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, however, 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 planimentum. And it was shown that 1 Ibid. vol. v. p. 261 (1888), and Bull. Soc. Geol. Nord, vol. xv. p. 114 (1889). 2 These ridges are present in the Mexican Loggerhead. |
