FOSSIL REPTILIA OF THE LONDON CLAY. 



the interspaces between their own vertebral body, and that of the preceding vertebra. 

 Tlie tenth vertebra supports a short pair of ribs in Chelone and in Eviijs, but not in 

 Trionyx; and this vertebra is commonly reckoned as a " lumbar" one. The eleventh and 

 twelfth vertebrae have short and thick ribs, which abut against the iliac bones, and they 

 are regarded as forming the sacrum. The remaining vertebrse belong to the tail, and are 

 "caudal." The costal plates articulate with each other, and with the neural plates by 

 fine dentated sutures. The free extremities of the ribs are implanted into sockets of 

 those marginal plates which are opposite to them. The 1st, 2d, 3d, and 10th, are not 

 so articulated in the loggerhead turtle. But all the marginal plates articulate with 

 each other, and with the nuchal {ch) and pygal {py) plates by sutures. 



The osseous basis of the plastron consists of nine pieces, one single and sym- 

 metrical, the rest in pairs. 



The median piece, 5, is the entostcrnal ; the anterior pair, es, is the episternal ; the 

 second pair, hs, the hyosternal ; the third pair, //s, the hyposternal ; and the posterior 

 pair, xs, the xiphisternal. 



With regard to the nature or homologies of these bones, three views have been 



Fig. 3. 



taken. The one generally adopted, on the authority 

 of Cuvier, Bojanus, and GeofFroy St. Hilaire, is, that 

 the nine bones of the plastron are subdivisions of a 

 vastly expanded sternum, or breast-bone; the second 

 view is^ that these subdivisions of the sternum are 

 enlarged by combination with ossifications of the in- 

 tegument ;* and the third view, in which Rathke 

 stands alone, is, that they are exclusively dermal 

 bones, and have no homologues in the endoskeleton 

 of other vertebrata.f 



Since this opinion is given as the result of that 

 celebrated embryologist's observations on the de- 

 velopment of the Chelonian reptiles, I have tested it 

 by a series of similar researches on the embryos and 

 young of the Chelone viydas and Testudo indica, and 

 have been led by them to conclusions distinct from any of the three theories above cited. 

 The sternum, like the carapace, is, without doubt, a compound of connate, endo- 

 skeletal and exoskeletal pieces ; but the endoskeletal parts are not exclusively the 

 homologues of the sternum. For the details of the observations, and the special 

 arguments on which these conclusions are founded, I must refer to my paper in the 

 ' Transactions of the Royal Society,' 1849; the homologies of the endoskeletal parts 

 of the plastron will require a brief illustration here from comparative anatomy. 



* Peters, Observationes ad Anatomiam Cheloniorum, 1838. 



t Ueber die Entwickelung der Schildkroten, 4to, 1848, p. 122. 



Bones of the plastron of the Loggerhead 

 Turtle {Chelone caouannd). 



