DERMATEMYDID.dE. 273 



hones ascend to the borders of the first costals. The inguinal buttresses do not reach the 

 upper borders of the peripherals. 



At Yale University is a specimen (No. 484) which was collected in 1870, at Millersville, 

 Wyoming, therefore in the lower third of the Bridger deposits. It furnishes a crusht carapace 

 and a complete plastron (text-fig. 338), the nearly complete skull (plate 37, figs. II, 12, 13), 

 and portions of" limb bones. The carapace has a length of 445 mm. and a width of 330 mm. 

 There is a large suprapygal 45 mm. long and 70 mm. wide. The first vertebral scute is 100 

 mm. wide; the second, 52 mm.; the third. 50 mm.; the fourth, 66 mm.; the fifth, 95 mm. 

 Besides these normal vertebrals there is a small supernumerary one, 37 mm. long, cut off from 

 the front of the normal fifth. The plastron (fig. 338) is 305 mm. long. As will be seen from 

 the figure, there are on each bridge 4 inframarginals. A small one appears to have been cut 

 off from the anterior end of the large third one of other specimens. In this specimen the 

 anterior extremity of the plastron comes forward as far as the front of the carapace; but this 

 is doubtless due to crushing. 



A specimen at Princeton University, No. 10074, is half as high as long. 



Dr. Leidy's second specimen, the one which furnisht the truncated anterior lobe of the 

 plastron, is in the U. S. National Museum and has the number 5000. In all respects, except 

 the form of this lobe, the specimen agrees with typical specimens of B. icyomingensis. The 

 plastron alone is represented and of this a portion of the hinder lobe is missing. The length 

 of the anterior lobe is 93 mm.; its width at the base 145 mm.; width at the epiplastral sutures, 

 1 14 mm. The entoplastron is 76 mm. long and 64 mm. wide. The width of the bridge is 108 

 mm. The hinder lobe has a width of 108 mm., diminishing in a curve to a width of 96 mm. 

 at the hvpoxiphiplastral sutures. 



Materials for the study of the skull are furnisht by the specimen at Yale University, 

 bearing the number 484 (plate if, figs. 11, 12, 13), and by number 5967 of the American 

 Museum of Natural History (plate 38, fig. 10). Each of these skulls is accompanied by its 

 shell. There appear to be no specific differences between them. The Yale skull is nearly 

 complete, the only portions missing being the roof of the orbits and of the nasal cavities. 

 The American Museum specimen supplies the parts missing in the Yale skull; but it is 

 defective in many respects. That at Yale furnishes the description here given, unless other- 

 wise specified. 



Relatively to the size of the shell the skull is small. It is broad behind and tapers forward 

 to a rather narrow snout. The length, from the snout to the occipital condyle, is 67 mm.; to 

 the extremity of the supraoccipital spine, 88 mm. The greatest width, taken at the upper 

 borders of the tympanic cavities, is 58 mm. The temporal region is not rooft over, and there 

 is no parieto-squamosal arch. The postorbital arch is slightly more than 7 mm. wide. The 

 zygomatic bar is deeply excavated on its lower border, and is 10 mm. wide. At the hinder 

 border of the orbit, where the maxilla joins the jugal, the former bone is only 5 mm. wide. 

 The interorbital space, as shown by the American Museum specimen, has a width of 23 mm. 

 The orbits are placed well forward. The antero-posterior diameter of each was about 20 mm.; 

 the perpendicular. 17 mm., the latter dimension being furnisht by the American Museum 

 specimen. This individual also shows that the nares had a perpendicular diameter of 1 1 mm., 

 a transverse diameter of 16 mm. The upper jaw is convex on its lower border, rising in front 

 so as to form a median notch. This upper jaw, as shown especially by the American Museum 

 specimen, has a sharp cutting-edge, but no masticatory surface. This appears to be an indi- 

 cation that this animal was not accustomed to crush shells, or other hard substances, but to 

 live on more active prey. 



The tympanic cavity has the longest diameter directed upward and slightly backward; 

 its outer rim is thick and heavy. The posterior wall is open, forming a notch for the passage 

 of the stapedial rod. The sutures between the bones of the skull are close and some are to be 

 traced only with difficulty. There seem to have been no nasals. As shown in the skull 

 belonging to the American Museum, the prefrontals run backward over the orbits and join 

 the postfrontals, excluding the frontals from the boundary of the orbit. From the individual 

 just mentioned the postfrontals are missing, but the sutural borders of the frontals and of the 

 parietals articulating with the postfrontals are well preserved. The jugal is a bone of con- 

 siderable length, and it forms a large part of the lower border of the orbit. 

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