BAENIDA. 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 g2 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 che 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 Hf ydromedusa. 
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 
premaxilla are wanting, but the length between these two was close to 60 mm. The greatest 
width (plate 14, fig. 1) Just in front of the ty mpanic cavities was 62 mm. From the tips of the 
nasals to the extremity of the supraoccipital i is 57mm. 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 18 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. 8g), 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 choanaz. The least width across the pterygoids i 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 1s 
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 aiece are represented in fig. 1 
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. rrparra. 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. r:paria 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 g mm. wide. In B. riparia the ridge along the junction of the basisphenoid and the 
pteryg goid is far less prominent. In this species, too, therel 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 ot 
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- 
