484 FOSSIL TURTLES OF NORTH AMERICA. 



Mexico. The exact locality is Coal Creek Canyon, in the southeastern corner of San Juan 

 County. The specimen bears the catalog number 6090. The skull is quite complete, with the 

 exception of the absence of the lower jaw. Figures of the object as seen from above, from below 

 and from the right side are presented. 



The skull is that of a large and evidently aged individual and no sutures are to be observed. 



The premaxilla and the anterior extremities of the maxillae are missing, but the missing 

 parts would add little to the length of the snout. The distance from the front of the nasal bones 

 to the extremity of the occipital condyle is 127 mm. The breadth across the zygomatic arches 

 is ill mm.; the distance from the outside of one quadrate to that of the other appears to have 

 been about 88 mm.; and the width across the skull at the middle of the tympanic cavities is 

 108 mm. It will be seen therefore that the roof of the tympanic cavity projects much over the 

 pedicels of the quadrates. Usually in living trionychids the breadth across the pedicels of the 

 quadrates is as great, or nearly as great, as the breadth at any other part. The hinder portion 

 of the skull is shortened. The extremity of the condyle is only 13 mm. behind the line joining 

 the quadrates. In a specimen of Platypeltis ferox 94 mm. long from the snout to the condyle, 

 the latter is placed at a distance of 17 mm. behind the quadrates. Relatively to other species 

 of the group the skull is broad, the breadth being 86 per cent, of the length. In Pelochelys 

 cantoris, whose skull is relatively broad, the width of the latter is 82 per cent, of the length. 

 The skull of the species under description is flat, and this appears to be due in no measure 

 to crushing. Placed on a level surface, the skull has its crest elevated to a height of 65 mm. 

 The snout was blunt. The skull maintains its breadth as far forward as the line joining 

 the posterior borders of the orbits, where it is 98 mm.; then the outlines converge rapidly. 

 The interorbital space is 15 mm. wide. The orbits look outward, forward, and upward, and 

 are circular, with diameters of 22 mm. From the orbits the sides of the face slope downward 

 and outward. 



The anterior nasal opening appears to have had little height, rising only 8 mm. above the 

 floor of the passage. Its breadth has been 20 mm. The lateral crests of the parietals converge 

 rapidly and meet opposite the fronts of the prootics. The postorbital arch has a width of 

 1 1 mm; the zygomatic arch a width of only 8 mm. in the middle of its length. At its anterior 

 inferior border it starts from the hinder extremity of the cutting-edge of the maxilla, and ascend- 

 ing passes backward to the front of the tympanic cavity. It is far more archt than that of 

 living trionychids. The tympanic cavity is relatively longer than that of Platypeltis ferox, the 

 length being 33 mm., one-fourth the length of the skull. In ferox the length of the cavity is 

 about one-fifth the length of the skull. 



In contradistinction to P. ferox, the upper border of the squamosal is not rolled down- 

 ward, but stands out as a sharp edge. Moreover, there does not appear to have existed 

 that long backwardly directed process of the squamosal and paroccipital which we find so 

 conspicuous in the living trionychids. It is possible that this process was originally somewhat 

 longer than now appears; but it could have been but little longer. 



The fossa inclosed by the zygomatic arch is greatly different from that of Platypeltis. 

 In the latter, and perhaps in all living members of the Trionychidae, the fossa is longer than 

 broad. In this Puerco species the length is 34 mm., the width about 38 mm. 



The occipital crest has been injured and is partly missing. It included a lower horizon- 

 tally expanded border and a thickened superior border. The latter may be, so far as preserved, 

 the backwardly prolonged parietals. 



The upper jaws were furnisht with subacute cutting-edges. From these the palate rises 

 in a high vault, 26 mm. above the cutting-edges. The choanae are removed far backward, 

 their front borders touching a perpendicular plane thru the posterior borders of the orbits. 

 The distance from the premaxillae to these choanae was about 50 mm. There was, therefore, 

 an enormous crushing surface on the upper jaws, and we may conclude with safety that the 

 animal was accustomed to devouring a prey that was protected by hard coverings, such as 

 mollusks. From each choana a groove in the roof of the mouth extends backward about3omm. 



The pterygoid region is only 36 mm. wide. In the specimen of Platypeltis ferox referred 

 to, the pterygoid region is eight-tenths the width of the upper jaws. This region in the fossil 

 is very concave transversely. The articulation of the quadrate was about 20 mm. wide. 



This skull is the most ancient one of the Trionychidae that is at present known. 



