WILLISTON: ON THE EXTREMITIES OF TYLOSAURUS. IO. 
the skin les intact in the region where the sternum should have 
been, so that it could hardly have been lost. There can then seem 
to be little or'no doubt that its absence in this genus, as in all the 
Tylosauride, is a fundamental character of the family, as I have 
previously defined it. Marsh has figured the coracoids of C/idastes 
as meeting in the middle line, but this is certainly an error. They 
are separated by a considerable expanse of cartilage as would in- 
deed be expected from the relationship to modern lizards. 
In the present specimen remains of the skin are found between 
the bones, from which it is evident that the membrane was very 
thin and phable and extended fully between the fingers to their 
tips. Small scute-like scales are found as far as the metacarpals, 
beyond which they are wanting everywhere, apparently. The 
numbers of phalanges in this specimen were apparently as follows: 
I-6, II-9, III-ro. [V—11, V-11. The distal one is preserved only 
in the fifth finger, and is, as is seen, very small and imperfect. | 
am much inclined to the opinion that the number of the phalanges 
is not always uniform in different individuals of the same species, 
though probably varying only within small limits. It will be ob- 
served that the fifth finger is longer by far than in either Platecarpus 
or Clidastes. In this, as in other respects, Platecarpus holds an in- 
termediate position between the two genera. Zy/osaurus is the 
least lizard-like of the American genera of the Pythonomorpha. 
The paddles are more slender, more flexible and relatively longer 
than in the other genera. 
The structure of the hind paddle, as shown incompletely in the ac- 
companying photographic illustration, (plate X) is of great interest, 
as proving, conclusively, I think, that there were five functional toes, 
though the fifth is evidently undergoing reduction and the first is 
not as long as in the front paddle. The femur is much more 
elongate than is the humerus. ‘The tibia is an unusually broad and 
flat bone; the fibula small and slender. In the front paddle only a 
single carpal bone is preserved, and I do not think that there were 
others in the living animal. It is a bony nodule evidently set in a 
plate of fibro-cartilage, and it is possible that in older individuals 
there may be additional ones. It is not at all unlikely that the 
same variations in the number of carpals and phalanges exists in 
this genus as does among the Cetacea. In the hind paddle the 
single tarsal preserved is of the same character as the carpal. 
Marsh has figured the hind paddle in P/atecarpus, and I have no 
doubt of its general accuracy. Dollo suspected that it might be 
wrong, and that the genus had but four toes, as in AZosasaurus. 
wt 
