CRETACEOUS FORMATIONS. 3 



chalk," and probably, therefore, from the Lower Chalk. Further evidence of the 

 remains of Chelonia in the cretaceous deposit is given in my paper on that subject read 

 before the Geological Society, April 29, 1840, and published in vol. VI, p. 411, of the 

 Second Series of the ' Geological Transactions.' The Chelonite there described and 

 figured was obtained from the Lower Chalk at Burham, in Kent, and consisted of 

 four marginal plates of the carapace, and a few other obscure fragments, sufficient to 

 prove that the species was not of a Trionyx or Tesiudo ; and as they diflfered in form 

 from those of the recent species of Chelone, with which I compared them, and 

 resembled rather the posterior marginal plates of some Emydians, I stated that this 

 correspondence " rendered it probable that these remains are referable to that family 

 of Chelonia which live in fresh water or estuaries." Subsequent observation of the 

 various interesting modifications by which extinct Chelones diminish, as it were, the 

 gap between the marine and fresh-water genera as they remain at the present day, 

 weakened the impression which the character of the marginal plates of the chalk 

 Chelonite first made in favour of its Emydian affinities ; and the examination of the 

 beautiful Chelonite, obtained from the same quarries at Burham, in Kent, and relieved 

 from the chalk matrix by Mr. Bensted, described and figured by Dr. Mantell in the 

 'Philosophical Transactions' for 1841, demonstrated that it is not an Emys but a true 

 Chelo?ie, as I have stated in the note appended to my paper in the ' Geological 

 Transactions.' 



As one of the figures in Dr. Mantell's Memoir, PI. 12, fig. 2, exhibits the extra- 

 ordinary character of ten pairs of ribs in the carapace of this rare fossil, permission 

 w^as obtained for original drawings to be made from the specimen, and these form 

 the subjects of T. I and T. II of the present Monograph. 



From the time of Caldesi,* the constancy of the number of pairs of ribs 

 which enter into the formation of the carapace of the Chelonian Reptiles has been 

 confirmed by all subsequent observations. No anatomical fact, perhaps, is better 

 determined, and more plainly and positively laid down, in all handbooks of Comparative 

 Anatomy. Perhaps no monstrosity would sooner arrest the attention, or excite more 

 wonder in the Comparative Anatomist, than the appearance in a recent or fossil 

 Chelonian of a greater number of pairs of ribs in the carapace than 8. When, there- 

 fore, I saw the figure 2 of Plate XII of the volume of the ' Philosophical Trans- 

 actions' for the year 1841, exhibiting not fewer than 10 expanded ribs on the left or 

 entire side of the fossil carapace, and 9 expanded ribs on the mutilated right side of 

 the same carapace, and found the experienced and well-known author appealing! to 



* Osservazioni anatomiclie intorno alle Tortarughe maritime d'Acque dolce et Terrestre ; 4to, 1687. 



t Dr. Mantell's words are — "The inner surface of the carapace is also thus displayed (PL 12, fig. 2), 

 together with the mode of union and growth of the costal processes, and the attachment of their distal 

 extremities to the osseous border. The accuracy of the drav'ings renders any detailed description un- 

 necessary."— Phil. Trans., 1841, p. 156. 



