R&PTILIA: LIMNOSCELIS 45 



the fourth toe severally connected by matrix, but not positively 

 attachable to the basal bones of their respective digits, because of 

 the effacement of the matrical surfaces in collecting. That they 

 belong with these toes is, however, beyond doubt, both because of 

 their perfect anatomical association and the peculiarities of the 

 matrix. The formula as is thus seen is, like that of the front feet, 

 the primitive one for reptiles, 2, 3, 4, 5, 4. The phalanges, as of 

 the front feet, are all remarkably short and broad, and I may also 

 add, relatively thin. The ungual phalanges, as have been described 

 for Diadecfes, which they resemble, are short, broad and hoof -like 

 rather than claw-like, with a thin rounded extremity, the bones 

 possibly incased in a horny nail in life. I can hardly conceive of a 

 foot of this character being used for burrowing, notwithstanding 

 Case's comparison of the similar feet of Diadectes with those of the 

 gopher. The right front foot, as preserved in the matrix, had the 

 tibia and fibula, with their attached proximal carpals, pressed down- 

 ward somewhat below the proximal ends of the metatarsals, but 

 not a vestige is preserved in the matrix of centrale or tarsalia, nor 

 is there any tarsal bone preserved with either specimen, save the 

 four sets of proximal ones. It is not at all impossible, however, 

 that vestigial, nodular tarsalia may have been ossified, but it is not 

 very probable that they were. Chondrification was evidently here 

 a specialization, and in accordance with the almost universal rule 

 among terrestrial vertebrates we should expect that the process 

 would develop more rapidly in the hind than in the front feet. 



As bearing upon the probable aquatic adaptation of the carpus 

 and tarsus of Limnoscelis and their chondrification, it will be of 

 interest to compare the several stages of evolution in their pro- 

 gressive adaptation to aquatic life in the American genera of 

 mosasaurs, figures of which will be found in my work upon the mosa- 

 saurs in the fourth volume of the University of Kansas Geological 

 Survey. In Clidastes (Plate XXXIII) it will be observed that the 

 proximal row of carpal bones is fully ossified, all four of them large, 

 the radiale largest of all. The centralia have disappeared, or what 

 is less probable have united with adjacent bones. Of the carpalia 

 the first and fifth have disappeared, leaving the second, third, and 

 fourth of nearly equal size. In the restorations the fifth and first 



