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Show 64 MR. R. LYDEKKER ON THE [Feb. 19, family distinction. The present writer, in the communication laid before the Geological Society to which allusion has been made above, has, however, pointed out that there is such a complete transition from Lytoloma to Thalassochelys, that it appears impossible to justify the family separation of the extinct types. Turning to the palatal aspect of the specimen, which is figured two thirds of the natural size in Plate VI., and comparing it with the skull of Thalassochelys, one of the first points which strikes the observer is its extreme shortness, the width at the widest part of the temporal arch being exactly equal to the length from the occipital condyle to muzzle ; whereas in the Loggerhead the former diameter is considerably less than the latter, whilst in Chelone the difference between the two diameters is still greater. Still more noticeable is the backward position of the posterior nares, which are situated at a point one third the distance from the condyle to the muzzle, as indeed is mentioned in M . Dollo's description of the Belgian specimens. In that description it is, however, stated that the boundary of the posterior nares is formed by the development of palatal plates from the pterygoids. So far, however, as can be seen from the present specimen, it would appear that this border is really constituted by the palatines, since on either side there seems to be a distinct suture separating the bones forming the border of the posterior nares from the undoubted pterygoids. Looking at the arrangement of the palatines in the Loggerhead, it would seem much more natural that these should be prolonged backwards, rather than that the pterygoids should assume the condition assigned to them by M . Dollo. In either case the vomer is excluded from the posterior nares, but its position anteriorly is not shown in this specimen. The pterygoids themselves are comparatively short, and much more deeply emarginate laterally than in the Loggerhead, in which respect they agree with those of the genus Argillochelys, which I have recently proposed ** for the reception of Chelone cuneiceps, Owen, of the London Clay. The palatal apertures of the temporal fossa are relatively large, and were probably nearly or quite as wide as long, in which respect they would also agree with Argillochelys, while they are not very widely different from Thalassochelys. The V formed by the inferior border of the presphenoid is wider and lower than in the Loggerhead, and more nearly resembles the same part in Argillochelys. Turning to the occipital aspect of the skull, as shown in Plate VII., it will be seen that the general contour and arrangement of the individual bones is so essentially the same as in the Loggerhead, as in the writer's opinion to be absolutely conclusive that the two forms should be placed in the same family. The similarity between the two is especially marked in respect of the quadrate and the bones surrounding the foramen magnum, and also in the contour of the channel for the stapes (columella). In Chelone the channel for the stapes is very deeply seated and short, but it becomes shallower and longer in Thalassochelys; and in the present form it is still less deep, 1 Quart. Journ. Geol. Soc. vol. xiv. pt. 2 (1889). |
