OCR Text |
Show 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 Thalassochelys in the contour of the aperture leading into the labyrinth 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 tbe 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 indicating 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 |