58 The Ottawa Naturalist. [Aug. -Sept. 



New York, at Corral Draw, in Ziebach county, South Dakota'' 

 . . . . "in a block of limestone which had come from the Lower 

 Oreodon beds." A comparison of the imperfect anterior lobe 

 of the plastron of T. thomsoni, the only portion of the shell of 

 this species known, with the corresponding part of the plastron 

 of the Sage creek specimen discloses in the latter the much greater 

 proportionate size of the epiplastral lip and differences through- 

 out in the proportions of the elements forming the lobe. 



The Sage creek specimen consists of the carapace and 

 plastron in a good state of preservation. The sutures between 

 the bones are clearly seen and the sulci indicating the boundaries 

 of the scutes are distinct. The carapace has been injured in 

 the neighbourhood of the eighth neural, and that bone is missing. 

 The plastron has been crushed in between the inguinal notches 

 with the result that in this portion of the shell the sulci and 

 sutures have been partly destroyed. The fractured humerus of 

 both of the fore limbs protrudes from the rock which fills the 

 axial notches, and the distal ends of the tibia and fibula of the 

 left hind leg are exposed in like manner behind. 



The carapace has been slightly flattened and its breadth in 

 this way increased to some extent, but as it now is the maximum 

 breadth is 410 mm. and the greatest length 479 mm. It rises 

 120 mm. from its lateral margin to the centre. Viewing it from 

 above, its breadth decreases more rapidly forward than back- 

 ward, so that the posterior is greater than the anterior half and 

 has a squarer outline. This is caused mainly by the enlargement 

 of the peripherals above the hind limbs. The lateral peripherals 

 and the pygal continue the general convexity of the shell down- 

 ward at an increased angle; the peripherals in front are produced 

 almost horizontally forward toward their free edges whilst the 

 more posterior lateral ones are only moderately inclined. 



The nuchal bone measures 90 mm. along its free margin in 

 front. It broadens slightly backward to its maximum breadth 

 of 103 mm. and then rapidly narrows to its full length of 95 mm. 

 It is notched behind to receive the front edge of the first neural. 



Of the eight neural bones the first is the longest ; it is much 

 longer than broad, narrows toward the front, and its outline is 

 convexly rounded in front and behind. The second is hexagonal 

 with the two short sides directed backward, and the front and 

 back edges incurved or concave. The third is quadrangular, 

 longer than broad, with its front edge convex and the hinder one 

 straight. The fourth, fifth, sixth and seventh are hexagonal 

 with their two short sides directed forward. 



There are two suprapygal bones of which the first is deeply 

 notched behind, with the forks in contact with the pygal and 

 the eleventh peripherals. Its surface is broadly convex in all 



