62 



The Ottawa Naturalist. 



[Aug. -Sept. 



As regards the peculiarities of structure in the carapace of 



this individual. The first costal scute is the largest, but its 



hinder sulcus has been crowded forward to some extent. The 



succeeding six are narrow antero-posteriorly, and take the place 



of the large second, third and fourth costal scutes which occur in 



nearly all fossil turtles with the exception of those belonging to 



the Trionychoidea. The first and second pair proceed outward 



on either side from the second vertebral scute, the third and 



fourth pair from the third vertebral, the fifth and sixth pair from 



the fourth vertebral, and the seventh pair from the accessory 



fifth vertebral. The relative position and size of the bones of 



the carapace and of their overlying scutes can be best understood 



by reference to plate III. The costal scutes correspond exactly 



on either side of the shell in each pair. The second, third, fourth, 



fifth, sixth and seventh measure distally, in an antero-posterior 



direction, 39, 26, 29, 26, 28 and 38 mm. respectively. The sulci 



between them pass down the second, third, fourth, fifth, sixth 



and seventh costal bones. The hinder sulcus of the seventh scute 



is on the eighth costal bone in advance of the latter's hind margin. 



The additional vertebral scute is small in comparison with the 



others, is broadest in front, and measures 28 mm. in length and 



44 mm. in breadth anteriorly. Its front sulcus crosses the 



seventh neural and the hinder one the accessory plate. 



The accompanying text-figure, 

 one-half the natural size shews 

 the shape relative size and 

 position of the bones and scutes 

 of the carapace in the neighbour- 

 hood of the additional vertebral 

 scute, the boundaries of the 

 scutes being indicated by the 

 heavy outlines. 



The seventh neural bone is 

 hexagonal, but differs from the 

 preceding hexagonal ones with 

 two short sides directed forward, 

 in having its four lateral margins 

 about equal. Both the eighth 

 neural and the accessory plate 

 are slightly irregular in shape. 

 The eighth has six sides, of which 

 two are short and are directed obliquely backward. The acces- 

 sory plate is hexagonal with two short sides toward the front, 

 and it joins the preceding eighth neural in an irregular manner, 

 as shewn in the figure. The eighth pair of costal bones are in 

 contact with both the eighth neural and the accessory plate. 



X. 7, X 8, neural bones 7, 8; 

 AP., accessory plat <; 

 C. 7, C. 8, C. 9, co: tal ties 7, 8, 9; 

 SP. 1, first suprapygal bone. 



