BAENID^S. 69 



bones the limits of the mesoplastrals can be pretty well determined. They appear to have 

 joined each other at the midline for a distance of about 25 mm. The hinder lobe is 84 mm. 

 long; 114 mm. wide at the base; and 92 mm. wide at the gulo-humeral sulcus. At the rear 

 there is a broad and shallow notch. The plastral scutes offer no especial deviations from the 

 normal. Their forms and dimensions may be determined from the figure. On each of the 

 bridges there are 3 inframarginals. 



The right humerus (fig. 47) lacks the distal end. Its length has been approximately 70 mm. 

 The resemblance to that of Chelydra is close. The head is, however, not so large, and especially 

 it is comprest in a plane perpendicular to the dorsal surface of the bone. Furthermore, the 

 plane of this comprest head inclines to the radial side of the humerus; whereas, in Chelydra, 

 it falls far outside of the ulnar side. The ulnar and radial processes are large and the distance 

 from the outside of one to the outside of the other measures 32 mm. In the case of the humerus 

 of the type of B. riparia this distance is only 26 mm. In B. arenosa, as well as B. riparia, the 

 planes of the two processes are at right angles with each other, as in Chelydra. The processes 

 are much thicker than in Chelydra. The shaft has a diameter of 8.25 mm. The groove 

 leading to the ectepicondylar foramen is broad and deep. A fragment from near the distal end 

 of the left humerus has a width of 21.5 mm. The right ulna (fig. 48) is present. The shaft is 

 not flattened like that of the Cryptodira, but is nearly cylindrical, like that of Hydromedusa. 

 The articulation for the humerus is, however, like that of the Cryptodira. The bone is 52 mm. 

 long; 15 mm. wide at the distal end. 



Plate 14, figs. 1-3, represents what is left of the skull. The occipital condyle and the 

 premaxillae are wanting, but the length between these two was close to 60 mm. The greatest 

 width (plate 14, fig. i) just in front of the tympanic cavities was 62 mm. From the tips of the 

 nasals to the extremity of the supraoccipital is 57 mm. All the sutures of the roof of the skull 

 are obliterated. The tympanic cavity (plate 14, fig. 2) has a horizontal diameter of 15 mm. and 

 a vertical of 1 8 mm. The stapedial rod remains in its natural position. The eyes appear to 

 have lookt upward and outward more than in Chisternon hebraicum (fig. 78, p. 89), and were 

 smaller. The orbits are circular with a diameter of 16 mm. The interorbital space is 22 mm.; 

 the nasal opening, 20 mm. wide; the maxilla, below the eye, 10 mm. wide. Seen from below 

 (plate 14, fig. 3), the cutting-edge of the maxilla is sharp, thickened upward to 5 mm., and at 

 least 30 mm. long. There is a prominent ridge on the triturating surface of the maxilla, highest 

 on each side of the choanae. The least width across the pterygoids is 13 mm. From the outer 

 posterior angle of the basioccipital bone a strong ridge runs forward toward the basisphenoid. 

 The basisphenoid did not come into contact with the vomer. Unfortunately, the lower jaw is 

 missing. 



On the upper side of the skull are seen numerous anastomosing grooves, the boundaries of 

 horny scutes that covered most of the upper surface. Some of these are represented in fig. I , 

 plate 14. 



The skull differs from that of Chisternon hebraicum in having a wider interorbital space, 

 a smaller eye, and far more prominent masticatory ridges on the upper jaws. It is most like 

 that of B. riparia. Unfortunately, both skulls are damaged, so that full comparison is not pos- 

 sible. The two skulls are of almost exactly the same size. The snout of B. arenosa was broader, 

 as was also the interorbital space. The eye of B. riparia appears to have had a greater vertical 

 diameter, but of this we can not be certain at present. In B. riparia the maxilla below the orbit 

 is only 9 mm. wide. In B. riparia the ridge along the junction of the basisphenoid and the 

 pterygoid is far less prominent. In this species, too, there is a low ridge running from the pedicel 

 of the quadrate to meet the free border of the pterygoid. Another, starting from the same 

 point, runs forward and upward along the suture between the quadrate and the pterygoid. 

 The surface of the bone between these two ridges is somewhat scoopt out. In B. arenosa the 

 ridges are little developt and the space between them is convex. 



No. 1115 of the American Museum is identified as belonging to the present species. It 

 was collected by the museum's expedition of 1893 into southwestern Wyoming. The locality 

 whence it was obtained is Laclede Meadows, southwest of Bitter Creek station and west of 

 Haystack Mountain. The deposits belong to the Washakie formation. Of this individual the 

 front and a part of the right border of the carapace (fig. 49) and the front of the anterior lobe 

 of the plastron (fig. 50) are missing. The remainder of the shell is well preserved. It had origi- 



