144 AMPHIBIA AND PISCES OF THE PERMIAN OF NORTH AMERICA 



account for the accumulation of large numbers of animals in small areas, 

 and it would also account for the attitude in which they are found. 



Measurements, mm 



Length of a skull on median line 20.8 



Length of a second skull, same 19.5 



An average vertebra 



Height from bottom of centrum to top of neural arch 7 



Greatest length of centrum 5 



Greatest length of neural arch 7 



Genus GYMNARTHRUS Case (p. 69). 

 Gymnarthrus willoughbyi Case (p. 69) . 



Characteristic specimens: The type and paratype Nos. 4892 and 4763 

 Am. Mus. Nat. Hist. 



The type skull is small, total length slightly over 16 mm., but, as the 

 premaxillaries are imperfect, it is impossible to give the exact length. At 

 first sight the skull resembles closely those described by Cope as Pariotichus 

 and Lysorophus, but it is radically different in the postorbital region. The 

 postorbital portion of the roof of the skull has been cut away from the 

 lower edge in the manner of some turtles, until the quadratojugal is gone, 

 the prosquamosal doubtfully present, and the quadrate fully exposed and 

 perhaps movable. The posterior portion of the skull is roofed over and 

 there is no suggestion of temporal openings. The quadrate is elongate and 

 of peculiar shape (see fig. 49); it articulates with the squamosal above and 

 seemingly with the occipital (supraoccipital + paroccipital + exoccipital) plate 

 behind. The posterior surface is somewhat injured and it is impossible 



Fig. 49. 



A. G. mlloughiyi. X |. No. 4763 Am. Mus. Lateral view of skull. 



B. Lower view of skull. Same specimen. 



C. G. willoughbyi. Lateral view of skull. No. 4892 Am. Mus, X |. 



to make out the form and relations of the various bones. The relations of 

 the bones of the upper surface are shown in fig. 16. The premaxillaries 

 were apparently large and sent back a process between the nares to meet 

 the nasals. The anterior nares were of good size and terminal in position. 

 The maxillary carries nine teeth; the posterior is small, the penultimate 

 the largest; from this point they diminish regularly toward the anterior 

 end. There are no tusks or enlarged teeth on the premaxillaries, the incisor 

 teeth sharing in the regular diminution in size toward the anterior end of 

 the skull. The three posterior teeth are the largest and have a greater 

 anteroposterior diameter than transverse. There Is no cutting edge on 

 the anteroposterior faces. 



On the lower surface the basioccipital occupies a small space at the 

 rear; the basisphenoid is a large fiat plate and unites directly with a strong 

 parasphenoid process. At the anterior end the parasphenoid meets two 

 diverging plates, which are apparently the palatines, but in large part they 

 are covered by the lower jaws. The lower edge of the pterygoids can be 



