94 AMPHIBIA AND PISCES OF THE PERMIAN OF NORTH AMERICA 



upper surface is somewhat injured by a transverse break. In general, the 

 position of the bones is as made out by Branson (2, fig. iia), but the course 

 of the sutures could not be exactly traced in his specimen. In plate 7 is 

 shown the lower surface of specimen No. 4673 Am. Mus., on which the course 

 of the sutures was traced in ink before the photograph was made. 



The premaxillaries show nearly as widely on the lower surface as on 

 the upper. On the right side, a posterior extension reaches nearly, but not 

 quite, to the anterior edge of the internal opening of the nostril; on the other 

 side it does not extend so far. There are no tusks on the premaxillaries, but 

 just within the tooth line and near the symphysis there is a deep pit in each 

 bone. The teeth on the anterior portion are small, but are slightly larger 

 near the middle of the bone. In the vicinity of the premaxillary suture the 

 teeth of both the premaxillary and maxillary are smaller. 



Fig. 27. — Eryops megacephalus. No. 117 Univ. of Chicago. 



A. Posterior view of skull, modified from Branson. X |. so, supraoccipital plate; i, tabulare; op, 



opisthotic; pt, pterygoid; psq, prosquamosal; }, quadrate. 



B. Inner view of lower jaw. After Branson. X i. d, dentary; /, splenial; e, coronoid; pa, prearticu- 



lar; 0, angular. 



The maxillaries extend back to a point opposite the posterior end of 

 the palatal vacuities. At the anterior end they form the outer edge of the 

 choanse and, uniting with the prevomers before and behind, complete the 

 border of the opening. Just posterior to the choanse they widen and are 

 separated from the palatal vacuities by a very narrow extension of the 

 pterygoid. On this widened portion and just posterior to the opening is a 

 slightly raised space bordered by an elevated rim; within the rim are the 

 bases of two tusks. It is apparent that one of these was functional, while 

 the other was absent or growing; the same thing is true of the tusks on the 

 palatine bone. It is perhaps noteworthy that the same condition occurs in 

 the double canine tusks of Dimetrodon, i.e., the posterior one is functional on 

 one side while the anterior one is functional on the other side, and vice versa. 

 There are two large teeth on the edge of the bone just opposite the tusks. 



Posterior to the tusks, the edge of the bone turns outwards, then back- 

 wards and obliquely forwards again, nearly to the edge of the skull; posterior 

 to this point the maxillary shows only as a very narrow edge on the skull. 

 From the point where the posterior edge of this part of the maxillary curves 

 around the palatine, a suture runs forward on the maxillary, but disappears. 

 It is present on both sides. There may have been a division of the bone in 

 youth, but the suture is incomplete in this specimen. 



The teeth are irregular in size, generally larger in the anterior parts of 

 the maxillary and premaxillary and becoming smaller posteriorly, but it 

 is noticeable that there is a great diversity in size all through the series; 

 this is probably due to the constant loss and replacement of teeth. The teeth 

 are conical, little recurved, and show a decided but simple unfolding of the 

 dentine. 



