750 PKOF. OWEN ON THE AFFINITIES OF THE MOSASATJKIILE. 



I subjoin (p. 749, fig. 4) a copy (reduced one half )of his reconstruc- 

 tion* of the skeleton of the fore limb of the species of Mosasauroid 

 which he calls Lestosaurus simus, which figure may be taken to be 

 sufficiently accurate to serve as a subject of comparison with the bones 

 of the fore limb of a Cetacean (ib. fig. 1), of a Plesiosaur (ib. fig. 2), 

 and of a Lacertian (ib. fig. 3). 



Prof. Marsh writes : — " This specimen clearly establishes the fact, 

 hitherto unknown, that the number of digits in the manus was 

 five, and that the longest digits had six phalanges, and the shortest 

 had but four. Moreover, that the paddle was expanded as in some 

 of the Cetaceans, and not contracted as in Ichthyosaurus, Plesio- 

 saurus, and other Enaliosaurs " (op. cit. p. 4). 



The author thus inclines to the Cetacean alternative of Cuvier's 

 view, whilst rejecting, with me, the Enaliosaurian one. As to any 

 approach to the Lacertian type in the framework of the fore limb of 

 his Mosasauroid Prof. Marsh is silent ; but he rightly recognizes 

 such approach in the structure of the coracoid element of the sup- 

 porting arch of that fin. 



In his enumeration of the phalanges Prof. Marsh reckons the 

 metacarpal as one ; in my copy of his figure (p. 749, fig. 4), I 

 connote these bones as in my figure of the Lacertian fore limb (fig. 

 3), homologizing the proximal row, in both limbs, as metacarpals. 



The first character to be noted in that limb of the Cetacean (ib. 

 fig. 1) is the sudden acquisition of length by the digit n and the 

 more or less gradual shortening of the digits in, iv, and v, — the 

 first digit, moreover, being chiefly represented by its short metacar- 

 pal bone, to which may be added, in some species, two small pha- 

 langes (e. g. Phoccena communis, Globiocephalus propinquus, Balcena 

 australis, Pontoporia tenuirostris), while Microptera bidens, Ziphius 

 cavirostris, and some other Cetacea have but one small pointed 

 phalanx added to the short metacarpal f : but no known Cetacean 

 deviates materially in reference to the present comparison from the 

 type figured (fig. 1). The digits increase in length from the ulnar to 

 the radial side of the fin, as far as the second digit. 



In Plesiosaurus the metacarpal of the pollex (i) may support three 

 phalanges ; the second digit (n), as in Cetacea, acquires a sudden 

 increase in length and number of phalanges, amounting to five or 

 six ; the third (in) and fourth (iv) digits slightly increase in leDgth, 

 each having six phalanges ; and the fifth digit (v) has seven 

 phalanges, although, from the less advanced position of the metacar- 

 pal bone, it seems not longer than the fourth digit. But the general 

 type is exemplified by an increase in length of the digits from the 

 radial to the ulnar side of the fin. The metacarpal must be added 

 in reference to the number of phalanges assigned to Mosasaurus by 

 Prof. Marsh. 



* Plate x. op. cit. (£ nat. size). 



t A. W. Malm, Kongl. Svenska Vetenskaps-Akadeiniens Handlingar. Ban- 

 det 9, No. 2, tafl. iv., v. 



