1889.] CHELONIAN GENUS LYTOLOMA. 61 



bridge, and figured by Sir Richard Owen in plate ix. of the 

 memoir cited, under the name of Chelone planimentum, the descrip- 

 tion of the newly revealed palatal surface appears worthy of a place 

 in the Society's ' Proceedings.' It is not, indeed, that the chief 

 features of this surface have been hitherto unknown, for they have 

 been described by M. Louis Dollo, of the Royal Museum of Natural 

 History of Brussels, upon the evidence of specimens obtained from 

 the Lower Eocene of Belgium, which are probably specifically 

 identical either with the present form or with the one described as 

 Chelone planimentum. Hitherto, however, M. Dollo has given no 

 figure of the cranium, and I doubt whether any of the Belgian 

 examples can be as beautifully preserved as the present one. 



It has long been seen that the Chelonians from the London Clay 

 described by Sir Richard Owen under the general term Chelone 

 included many forms which could only be retained in that genus by 

 employing that term in a much wider sense than that in which it is 

 understood by students of recent herpetology. And from the year 

 186/ onwards a number of generic terms have been proposed for 

 these and allied Chelonians from other deposits, which has resulted 

 in an unusually complex synonymy. The chief features of this 

 synonymy it is necessary to notice in some detail before proceeding to 

 the consideration of the specimen before us. 



In the year 1870, Prof. E. D. Cope, of Philadelphia, published 

 his well-known " Synopsis of the Extinct Batrachia, Reptiha, and 

 Aves of North America" \ containing descriptions of the remains of 

 Eocene Chelonians allied to the present form, which were arranged 

 under several generic names, of which some had been first published 

 at earlier dates. The names which it will be necessary to mention 

 are — Osteopyyis, dating from 1868 ", which was based on the evidence 

 of the shell; Euclastes, dating from the preceding year^, and 

 founded on the cranium ; Lytoloma (1870), based on the evidence 

 of the mandible ; and Pnppigervs (1870), which was applied to 

 several of the Chelonians from the London Clay described by Sir 

 Richard Owen, Chelone planimentum not, however, being among the 

 number. In the following year Prof. H. G. Seeley * proposed to 

 distinguish the last-named species under the generic name of 

 Glossochelys. Thus matters stood till the year 1886, when 

 M. Dollo ' described some Chelonian remains from the Lower 

 Eocene of Belgium, which he regarded as closely allied to Chelone 

 crassicostata and C. planimentum, and proposed to refer, together 

 with these and some other species, to a new genus under the name of 

 Pachyrhynchus. That name, however, as was pointed out in a 

 joint paper by Mr. G. A. Boulenger and the present writer ^ was 

 preoccupied ; and in the following year its author ^ proposed to 



1 Trans. Araer. Pliil. Soc. vol. xiv. pt. i. (1870). 



2 Proc. Ac. Nat. Sci. Philad. 1868, p. 147. 



3 Ibid. 1867, p. 39. 



* Ann. Mag. Nat. ffist. ser. 5, vol. viii. p. 227 (1871). 



= Bull. Mus. E. Hist. Nat. Belg. vol. iv. p. 130 (1886). 



^ Geological Magazine, dec. 3, vol. iv. p. 270 (1887). 



^ Ibid. vol. iv. p. 393 (1887). 



