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 tliat 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 tem- 

 poral arch being exactly equal to the length from the occipital con- 

 dyle to muzzle ; whereas in the Loggerhead the former diameter 

 is considerably less than the latter, whilst in Chelone the differ- 

 ence between the two diameters is still greater. Still more notice- 

 able 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. DoUo. In either case the vomer is excluded from the pos- 

 terior 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 ot the genus Argillochehjs, 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 Ai-gillochelys, 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. xlv. pt. 2 (1889). 



