46 AMERICAN PERMIAN VERTEBRATES 



digits are placed in immediate contact with the bones of the first 

 row; probably in life they were separated by a cartilaginous 

 interval. In Platecarpus (Plate XLIV), the radiale and pisiform 

 have disappeared, the intermedium and ulnare alone remaining 

 with the third and fourth carpalia, the second gone. In Tylosaurus 

 (Plate XL VIII) the ulnare alone remains of the carpus, and that 

 very small and nodular, though in other specimens a very small, 

 nodular intermedium is sometimes found. In Clidastes hyper- 

 phalangy is just beginning, no finger having more than two extra 

 phalanges; in Platecarpus hyperphalangy had progressed until four or 

 more may have been acquired in some of the digits, while in Tylo- 

 saurus as many as twelve phalanges are known in the longest fingers. 

 In the tarsus of Clidastes (Plate XXXVI) the co-ossified tibiale 

 and intermedium have their usual articulations, with only one of 

 the other tarsal elements remaining, probably the fourth tarsale. 

 In Platecarpus (op. cit., p. 166) we have the same bones, but all of 

 them reduced. In Tylosaurus (Plate L) the tarsus is not well 

 known, but, evidently as in the carpus, there is only one mesopo- 

 dial bone, the fibulare, and that is reduced; and hyperphalangy 

 here too is carried to its greatest extent among mosasaurs. 



Briefly, it is seen from these illustrations that specialization has 

 progressed in the mosasaur limbs in nearly equal pace in the front 

 and the hind ones, though, if Dollo is right, in Mosasaurus (and 

 Clidastes ?) the loss of the fifth toe behind indicates the usual greater 

 specialization of the hind feet over the front ones, as in terrestrial 

 animals, though we know that this is not always the case among 

 aquatic animals. Again, they show very conspicuously that, as 

 in the Thalattosuchia, specialization begins on the radial and tibial 

 sides, and progressively extends to the ulnar and fibular sides. Per- 

 haps this is the explanation of the greatly reduced size of the 

 radiale in Limnoscelis, as well as the reduced size of the tibiale and 

 fibulare, which have reached nearly the condition found in Plate- 

 carpus. And, as I have elsewhere stated, it is evident that Clidastes, 

 so far as the limbs are concerned at least, is the most generalized 

 genus known of the Mosasauria, Tylosaurus the most specialized. 



No indications whatever of ventral ribs are present in either 

 specimen. In their place, however, the whole ventral region was 



