LQ PROCEEDINGS OF THE NATIONAL MUSEUM. VOL, XXXIV. 
corresponding to these are found in many turtles of several families. 
Among the Cheloniide they are found in Hretmochelys and Chelonia,; 
among the Emydide, in 7rachemys, Pseudemys, and Batagur, all the 
species of the genus Z’estudo are provided with similar ridges; and 
the writer has found a well-developed masticatory ridge in an extinct 
trionychid genus from the Bridger Eocene of Wyoming. Such 
ridges, often strongly tuberculated, appear to have been developed 
as substitutes for teeth, lost probably by the ancestors of the earliest 
turtles; and these ridges probably mark modifications of food-get- 
ting and food-preparation somewhat as do the variations in the 
teeth of other reptiles. | 
The structure of the roof of the mouth in front of the choane is 
quite different in the two species here discussed. In the loggerhead 
(Plate VIII, fig. 1) the maxillee meet each other on the midline, below 
the vomer; in the bastard-turtle (Plate VIII, fig. 2) the maxille are 
wholly separated by the vomer. 
Some statements regarding the lower jaw have already been made. 
To these it may be added that the lower jaw of the bastard-turtle 
(Plate IX, fig. 1; Plate XI, fig. 4) is shorter, heavier, more strongly 
upturned toward the tip and more bent outward at the sides than in 
the loggerhead. ‘The bony alveolar surface (Plate IX, fig. 1; Plate 
XI, fig. 4) is more deeply channeled on each side, and there is, at the 
hinder end of the symphysis, a triangular elevation, corresponding 
to that already mentioned as occurring on the horny sheath of the 
jaw. 
The scutes which cover the upper surface of the skull of the 
bastard-turtle appear to differ somewhat from those of the logger- 
head. The frontal scute and those which join it at the sides and 
in front are alike in the two species. The fronto-parietal of the log- 
gerhead is much larger, nearly twice as long as the frontal, while 
the parietals are short, little, if any, more than half as long as the 
fronto-parietal. On the contrary, the fronto-parietal of the bastard- 
turtle is not much longer than either the frontal or the parietals. 
These scutes are not shown on the figures presented here. 
It appears that, in the case of the bastard-turtle, the head becomes 
relatively smaller as age comes on. In No. 29015 the length of the 
head is contained in the length of the carapace 3.6 times; in the large 
individual, No. 29323, the head is contained in the length of the cara- 
pace 4.6 times. The same statement is true regarding the loggerhead. 
In No. 29372, with carapace 453 mm. long, the length of the skull is 
108 mm.; therefore it enters the length of the carapace 4.2 times. 
In the large individual, No. 29013, the length of the skull is contained 
in the length of the carapace 4.75 times. 
As to the size attained by the skull of the loggerhead, the largest 
known to the writer is in the U. S. National Museum. The length 
