PROF. OWEN ON THE AFFINITIES OF THE MOSASATTRIDiE. 751 



In the Lizard {Monitor niloticus, fig. 3), the first digit (i) is the 

 shortest ; the second (n) shows a marked increase of length ; this 

 slightly augments in the third (in) and again in the fourth digit 

 (iv), while the fifth (v) becomes shortened, though not in the same 

 degree as the first. 



The general proportions of the five digits show a near equality of 

 the three middle ones (u, in, iv), which are the longest, and a 

 marked inferiority in the length of the first and fifth digits. The 

 increase in length is from the radial to the ulnar side, as far as the 

 fourth digit ; then stops more decidedly, the fifth digit being much 

 shorter, although the metacarpal is on the same level as the rest. 



Now this is the general character also of the digits of the fore 

 fin in the Mosasaurian, fig. 4. The three middle digits are sub- 

 equal, slightly gaining in length from the second (11) to the fourth 

 (iv); while the first (i) and fifth (v) are much shorter; and of 

 these the first is shorter than the fifth. The numerical formula of 

 the phalanges in the Monitor and majority of Lizards is 2, 3, 4, 5, 3 ; 

 in the Mosasauroid, as restored by Prof. Marsh, the formula appears 

 to be 3, 5, 5, 5, 4. But it must be remembered " that a few of 

 the distal phalanges were somewhat displaced ; " and a phalanx 

 is added, in dotted outline, to digit n in plate x. of the ' Memoir.' 

 It may be allowable to conjecture, if the bones of the fore fin of a 

 Mosasauroid should be discovered without the degree of displacement 

 of phalanges to which Prof. Marsh's specimen had been subjected, 

 that a nearer approach to the Lacertian formula may be demonstrated 

 than is shown in p. 749, fig. 4. 



This, however, is by no means required to form a judgment on 

 the main issue from the bones of the fore fin as restored by Prof. 

 Marsh, viz. that the type of such fin is Lacertian rather than 

 Plesiosaurian or Cetacean. The differences in the latter types are 

 too great to need further remark. 



I may here, however, refer to a statement by Prof. Cope, bearing 

 upon another element in relation to the affinity of Mosasaurus, viz. 

 that the figure 23, p. 709, of my paper " On the Rank and Affini- 

 ties in the lieptilian Class of the Mosasauridce " * is copied from " a 

 figure of a vertebra of Clidastes stenops, from which the zygosphene 

 has been accidentally broken away "f. 



It will be seen, in referring to plate xviii. of Cope's great work 

 on ' The Vertebrata of the Cretaceous Formations,' 4to, 1875, that, 

 in selecting fig. 4 a to illustrate my comparison with the vertebrae of 

 Python, fig. 24, and Iguana, fig. 22 of my paper, I took the least 

 mutilated of the figures in Prof. Cope's plate, and one that gave no 

 indication on the side figured of any fracture or loss of parts. My 

 copy is exact, and the fault is with Prof. Cope's artist. The Pro- 

 fessor refers to several other figures, all of more or less mutilated 



* Quart. Joum. Geol. Soc, November 1877. 



t 'Bulletin of the United-States Geological Surrey.' rol. ir. no. 1, p. 300, 

 February 1878. 



Q. J. G. 8. No. 135. . 3 d 



