ADVERTISEMENT. 



When my friend Professor Bell consented to the announcement, by the 

 PALiEONTOGRAPHiCAL Society, of a Monograph by himself on Fossil Reptiles of 

 the London Clay, he was not aware of the progress already made in my woi'k on the 

 Fossil British Reptiles. As soon as he saw the proofs of the plates of those from the 

 London Clay he renounced his intention ; but, on the completion of the arrangement 

 with the Council of the Society for the publication of my work, he kindly consented 

 to allow his name to be associated with mine in the present Monograph, to which he 

 has contributed a very valuable and important share. The aid, however, which 

 Professor Bell has rendered has not ended with the descriptions which bear his initials. 

 By his careful revisions of those contributed by myself their accuracy has been 

 assured ; and I should not have dismissed the sheets containing the determinations of 

 the parts of the complex skeleton of the Chelonia, and the nomenclature applied to 

 them, with the confidence which I now feel, if they had not received the sanction of 

 an anatomist and naturalist who had distinguished himself in a comparatively early 

 part of his scientific career, by his beautiful Monograph on the existing species of the 

 Chelonian order of Reptiles. 



It only remains to add, that we have mutually co-operated in ensuring the accuracy 

 of the illustrations, and have been most ably seconded by the experienced artists 

 Messrs. Dinkel and Erxleben, to whom we beg to offer our best thanks. 



Our acknowledgments to the kind friends who have liberally submitted 



their specimens of Eocene Chelonia to our examination are expressed in the text, in 



which those specimens are respectively described. 



R. O. 



