CRETACEOUS FORMATIONS. 1.7 



the posterior border is convex, showing that it was not united in its whole extent to 

 the corresponding anterior border of the hyposternal. 



With the broad nuchal plate (cA) is articulated the first marginal plate m 1, of the 

 right side : its upper surface is square, and is impressed by the junction of the first 

 costal scute with the second and third marginal scutes. The second marginal plate 

 is lost. The third is displaced, and its concave side is turned upwards : the upper 

 and under walls of the concavity are of almost equal extent, and meet externally at 

 a right angle. Unless the back part of this plate has been turned forwards, it differs 

 from the corresponding plate in the Emydians in not having the inner concavity con- 

 fined to the posterior part, but extending its whole length, as in Thalassians ; its 

 proportions, however, are such as we find in the genus Bv/t/s. The fourth marginal 

 plate, m 4, has its inferior and superior walls equally produced, as in Emydians, and 

 meeting at a right angle : it articulates with the second costal plate, and probably, 

 also, with the hyosternal below, but it has been displaced upwards. The fifth 

 marginal plate is lost. Only the outer margin of the sixth, «z fi, is produced; this 

 also shows an upper and an under plate meeting at a right angle. The seventh 

 marginal plate, m 7, which is preserved on the left side, although fractured, shows its 

 rapid progressive compression towards its posterior border. The eighth marginal 

 plate, »2 8, is a broad, subquadrate, depressed plate, with a thin outer margin, and 

 the thicker inner margin slightly produced into the angle between the fifth and sixth 

 costal plates : its upper surface is concave, and impressed with the T-shaped union 

 of the third costal scute with the eighth and ninth marginal scutes. The ninth 

 marginal plate, m 9- presents a similar form ; its outer border is injured. In the 

 tenth marginal plate, m lo, the impression of that border is left on the matrix, showing 

 that it had an angular notch. The same character is as strongly marked in the 

 eleventh marginal plate, m ii, and the pygal plate, as has been already observed is 

 notched at the middle of the posterior border. It is from the consequent serrated 

 character of the hinder border of the carapace that the specific name has been taken. 



Compared with the existing species of the genus Chelone, the present fossil differs 

 greatly in the completeness of the ossification of the carapace, due to the extension of 

 the costal to the marginal plates : in the form and proportions of the marginal plates, 

 especially from the first to the seventh inclusive ; and in the form of the recognisable 

 elements of the plastron, more particularly in the curved and produced angle of the 

 hyosternal. But when we compare it with some of the extinct Turtles of the Eocene 

 epoch, as e. g., Chelone longiceps, Chelone convexa, and Chelone subcarinata, the differ- 

 ence in regard to the extent of ossification of the costal plates is less ; whilst the 

 persistent partial want of union between the elements of the plastron in the present 

 fossil, approximates that part of its skeleton to the condition of the plastron in the 

 Eocene Chelones above cited, in which the ossification of the plastral elements has 

 proceeded further than in the typical Turtles. 



3 



