No. 1747. DESCRIPTIONS OF W.'U Fossil. TURTLES HAY. 325 



length and a part of the hinder border near the distal cud. Originally 

 the costal had a length of about 180 mm. The breadth at the 

 neural end is 41 mm; at the middle of the length, 51.5 mm. The 

 thickness where the bone joined the neurals is 7 mm.; at the middle 

 of the length, on the front border. 13.5 mm.; ou the hinder border, 

 10mm.; through the ridge formed by the rib at the distal end, 11 mm. 

 The greater thickness of the anterior border, in the middle of the 

 length, is due to the fact that the rib lies ou the anterior half of the 

 inferior surface. The free border of the carapace seems to have 

 been cut off nearly at right angles with the upper surface, not beveled 

 off as in many species of the family. 



It is in the sculpture of the upper surface that is found a character 

 which appears to distinguish this species from its relatives. The 

 upper surface is furnished with a system of ridges which rise quite 

 abruptly from the nearly plane intervals between them. Usually in 

 the trionychid tortoises the ridges anastomose so as to produce pits 

 more or less regular in form and size. In the present species the 

 ridges show little tendency to anastomose, and on the proximal 

 two-thirds of the costal there are comparatively few distinctly 

 inclosed pits. On the proximal third the ridges run in no predomi- 

 nant direction and are interrupted and usually short. Many sepa- 

 rate little hillocks are present. 



On the median third of the costal the ridges, about 3 mm. apart, 

 run mostly at right angles with the intercostal sutures and there are 

 long flat valleys between them, but the ridges are often broken up 

 into rows of hillocks. On the distal third of the costal the ridges 

 are more irregular in their courses and are more often connected by 

 cross ridges, so that there are definitely formed pits. These become 

 more reduced in size as the free border is approached. Over the 

 whole surface, but somewhat less conspicuously on the ridges, are 

 seen the openings of minute vascular canals. 



