1889.] CHELONIAN GENUS LYTOLOMA. 65 
and appears merely to form the base of the valley formed by the 
sloping surfaces of the upper and lower halves of the quadrate. 
There is, however, a marked difference between the present form and 
Lhalassochelys in the contour of the aperture leading into the laby- 
rinth of the ear. Thus in the latter genus this aperture forms a 
long slit between the upper and lower bars of the exoccipital and 
opisthotic ; whereas in the present form there is only a very slight 
notch in the exoccipital, and scarcely any production of the portion 
below the notch, so that the contour of the aperture in question is 
heart-shaped. A similar condition obtains in Argillochelys. In 
the contour of the tympanic ring, as seen from the lateral aspect, 
the present form differs from the Loggerhead in the more downward 
direction of the quadratic bar of the quadratojugal and in the more 
sudden deepening of the anterior wall of the cavity. Here also the 
form under consideration agrees with the other extinct genus from 
the same deposits. 
The flatness of the inferior surface of the mandibular symphysis 
is well shown in this specimen ; while there is an equally clear 
display of the unusual depth of the masseteric fossa, as indicated by 
the great prominence of the ridge forming its inferior border. In 
his description of the mandible of the type species of Lytoloma, 
Prof. Cope lays great stress on this characteristic feature of the 
masseteric fossa, as indicative of great biting power. 
In conclusion, it appears from the study of the skull that Lytoloma 
should be regarded asa specialized modification of a generalized type 
of Chelonian, of which the nearest exsisting representative is to be 
found in Thalassochelys. Its close relationship in the characters of 
the skall with Argillochelys, in which the posterior nares have the 
same approximate position as in TAalassochelys, binds all the three 
genera into a single group, and indicates that the peculiar position 
of the posterior nares in Lytoloma cannot be looked upon as indi- 
cating more than a generic difference. 
Postscript. 
Since this paper was read I have come across a specimen in the 
British Museum (no. R. 918), from the London Clay of Harwich, 
which shows the associated cranium and carapace of this form, and 
which is therefore of extreme importance, since it enables us to 
confirm the reference of the cranium forming the subject of this 
communication to the so-called Chelone crassicostata, which, as I 
have already mentioned, was founded upon the evidence of the 
carapace. 
The specimen in question has been long in the Museum, where it 
was merely entered as a carapace. Upon close examination I 
detected, however, at the anterior extremity what appeared to bea 
portion of the skull, and by careful development Mr. Hall has 
succeeded in showing the greater portion of the frontal aspect of the 
entire skull. 
Now this skull, although considerably smaller, agrees in all 
Proc. Zoor. Soc.—1889, No. V. 5 
