144- NOTE ON A CHELONIAN SKULL. 



the sutures betAveen the constituent bones are scarcely distin- 

 guishable; but it is clear that the nasals (#.) and prefrontals 

 (prf.) are separate elements, arid that the latter are very small, 

 not meeting in the middle line, but allowing the former to unite 

 in suture with the frontals (_//%) The parietals (pa.} seem to be 

 relatively large, meeting the frontals in a transverse suture 

 between the hinder border of the orbits. The surface of all 

 the bones is roughened by a fine network of smooth ridges, 

 which are not very prominent. 



This skull is exactly such as might be expected to belong to 

 Pleurosternum, and its external ornament agrees so closely with 

 that of the common shells from Swanage that it may probably be 

 referred to the same genus. As already known, l Pleurosternum 

 exhibits some remarkable resemblances to the water-tortoises of 

 the family Chelydidae, which now live in Australia and South 

 America, and the new skull agrees with that of some genera 

 of this family in the peculiar arrangement of the nasal and 

 prefrontal bones. All the Jurassic Chelonian skulls hitherto 

 discovered, including one from the Portland Stone, 2 are closely 

 similar to that now described. 



1 R. Lydekker, ' ' On Certain Chelonian Remains from the Wealden and 

 Purbeck," Quart. Journ. Geol. Soc., Vol. XLV. (1889), p. 518. 



2 R. Owen, " History of British Fossil Reptiles" (1884), Chelonians, pi. viii., 

 figs. 1-3 (Chelone planiceps}. Named Stegochchjs by R. Lydekker, Quart. Journ. 

 Geol. Soc., Vol. XLV. (1889), p. 229. 



