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 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 trom the undoubted pterygoids. Looking at the 
arrangenient 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 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 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 WV 
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 VIL, 
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 shailower and 
longer in Zhalassochelys ; and in the present form it is still less deep, 
1 Quart. Journ. Geol. Soc. vol. xlv. pt. 2 (1889). 
