THALASSEMYDID&. 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 of L. angusta being 52 mm. In the latter species the hinder end of the symphysis 
falls 12 mm. behind the line joining the mental foramina; in E. molaria 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 they belonged to the 
same species, the angle of the jaw of the type of 
R. platyops ought to be the larger. Again,in case 
the present jaw had belonged to R. platyops, 
the hinder end of the symphysis would have 
AEE fallen a distance behind the choanz equal to the 
Fics. 198 AND 199.—Erquelinnesta molaria. length of the palatal plate of the vomer. The 
Lower jaw forming the type. 3. 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 choanz, thus interfering with breathing. 
From both Lytoloma angusta and L. wieland: this species differs in the greater length of 
the symphysis and in the smaller angle between the borders of the jaw. 
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 maxillz, the palatines, and the vomer. Choane near the middle of 
the roof of the mou h:; the palatines not meeting behind the vomer. A pit between the pre- 
maxillz for the reception of the upturned beak of the lower jaw. Shell and limbs unknown. 
Type: Euclastes platyops Cope. 
This genus differs from the species of Lytoloma (to which genus it has been referred since 
the discovery that the name Euclastes 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 premaxilla and an interruption of the bone. 
Similar conditions are found in the skull of Macrochelys. Osteopygis, Lytoloma, and F rquelin- 
nesia 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. 
Dy 
