thalassemydida;. 



161 



This jaw (figs. 198, 199) is remarkable for the great length of the symphysis, this length 

 considerably exceeding that of Lytoloma angusta. The width of the jaw, at the mental fora- 

 mina, is 88 mm.; at the hinder ends of the dentaries, 100 mm., dimensions almost exactly those 

 of the jaw belonging with the type of L. angusta. The length of the symphysis is 65 mm., that 

 of the jaw ot /.. angusta being 52 mm. In the latter species the hinder end of the symphysis 

 tails 1 2 mm. behind the line joining the mental foramina; in E. molana it falls 22 mm. behind 

 that line. The triturating surface is flat to near the inconspicuous cutting-edges, to which 

 it ascends. The lower surface of the jaw also is flat, ascending rather abruptly to the cutting- 

 edges. Anteriorly the lower surface approaches gradually the upper surface at the sharp- 

 edged tip of the jaw. The outline of this tip 

 is truncated and about 20 mm. wide. The 

 coronoid process rises about 33 mm. above the 

 bottom of the ramus, and the descent from it 

 toward the tip of the jaw is gradual. The 

 masseter fossa is large. 



The greatest thickness of the triturating 

 surface is near the hinder end of the symphysis. 

 The angle between the borders of the jaw, in 

 front, is considerably more than a right angle. 

 This jaw can not be that of Rhetechelys 

 platyops (Cope) for at least two reasons. The 

 angle between the lateral borders is too great 

 to fit the upper jaw of R. platyops. The type 

 of the latter species belonged to an individual 

 about twice as large as the possessor of the jaw 

 here described; and, if the}' belonged to the 

 same species, the angle of the jaw of the type of 

 R.platyopsought to be the larger. Again, in case 

 the present jaw had belonged to R. platyops, 

 the hinder end of the symphysis would have 

 fallen a distance behind the choanae equal to the 

 length of the palatal plate of the vomer. The 

 results of this would have been that a large part 

 of the triturating surface of the lower jaw would 

 have had no surface to oppose it and it would 

 have been applied against the choanae, thus interfering with breathing. 



From both Lytoloma angusta and L. uuielandi this species differs in the greater length of 

 the symphysis and in the smaller angle between the borders of the jaw. 



Figs. 198 and iqg. Erquelinnesta malaria. 

 Lower jaw forming the type. Xf. 



198. Upper view of jaw. 199. Side view of jaw. 



Genus RHETECHELYS nov. 



Skull broad and deprest. Temporal region widely rooft over. Triturating surface of upper 

 jaw broad, involving the maxillae, the palatines, and the vomer. Choanae near the middle of 

 the roof of the mou h: the palatines not meeting behind the vomer. A pit between the pre- 

 maxillae for the reception of the upturned beak of the lower jaw. Shell and limbs unknown. 



Type: Euclastcs platyops Cope. 



This genus differs from the species of Lytoloma (to which genus it has been referred since 

 the discovery that the name Euclastcs is preoccupied) in having had the tip of the lower jaw 

 furnisht with an upturned beak. The existence of this is inferred from the presence of a deep 

 depression between the palatal plates of the premaxillae and an interruption of the bone. 

 Similar conditions are found in the skull o{ Macrochelys. Osteopygis, Lytoloma, and Erquelin- 

 nesta all have lower jaws with broad, flat triturating surfaces, but in none of them do we find 

 any traces of a beak. 



The only known species of the genus was a large and powerful animal, whose skull equaled 

 in size that of the largest specimens of the living loggerhead. 

 I I 



