1889.] CHELONIAN GENUS LYTOLOMA. G5 



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 

 Thalassochelys 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 as a 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 skull with Argillochelys, in which the posterior nares have the 

 same approximate position as in Thalassochelys, 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 be a 

 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. Zool. Soc— 1889, No. V. 5 



