200 



FOSSIL TURTLES OF NORTH AMERICA. 



collection or of the collector. There is no doubt, however, that the specimen was obtained 

 from the Niobrara deposits of Kansas. It consists (fig. 256) of the greater portion of the 

 plastron, 12 peripherals, 2 or 3 neurals, and portions of 2 costal plates. With these are some 

 other bones which are either undetermined or they doubtfully appertain to the specimen. 

 The hyoplastra and hypoplastra closely resemble those of Protostega gigas. The two of 

 these bones which are found on the same side were apparently united for a short distance by 

 a coarse suture. The two hyoplastra evidently came close together along the midline; but 

 the hypoplastra were widely separated. The median fontanel has therefore extended from 

 the hyoplastrals to the xiphiplastrals. The latter bones are not so elongated as in modern 

 sea-turtles and Chelydra, nor so short and abruptly incurved as in Protostega gigas. The 

 greatest thickness of the hyoplastrals is about 10 mm. The other plastral bones are thinner. 

 There are no longitudinal carina? on the plastron, such as we find on that of Caretta caretta 



Figs. 256-259. Protostega advena. Portions of type. X^. 

 256. Plastron and peripherals, seen from above. 257. Two neurals. 258. Costal bone. 259. Postfrontal. 



and Protostega gigas. The forms and positions of the various bones may be seen from the 

 figures. The distance from the anterior border of the hyoplastron to the hinder border of 

 the xiphiplastron is 278 mm. The entoplastron and the epiplastra are wanting. 



The anterior peripherals of both sides are absent from the materials. On the right side 

 the most anterior peripheral present appears to be the one which has received the rib-end of 

 the first costal plate. This is provisionally regarded as the fourth from the nuchal bone. 

 The most anterior one on the left side is the fifth. On the left side one is believed to be missing 

 next to the pygal; while on the right side three in front of the pygal are supposed to be gone. 

 These peripherals prove that this individual is not a young Protostega gigas, since they are 

 relatively much shorter and broader than are the corresponding bones of the latter species. 

 They are likewise much heavier bones. 



