58 Prof. E,. Owen on the Occurrence 



The general adaptive characters of the Mosasaurian verte- 

 bra? relate to aquatic life ; and in the category (type 7) 

 in which the zygapophyses disappear, not to return, there is 

 a significant indication of a tendency towards the Cetacea ; in 

 a great extent of the caudal series (type 11) a piscine charac- 

 ter prevails. Both these modifications are engrafted on, or 

 associated with, the lacertian type of the procoelian vertebra. 



The movements of this gigantic reptile among the sea-waves, 

 mainly executed by the long and flexile vertebral column, 

 were guided or modified by the action of both pectoral and 

 pelvic fins : the retention of both pairs is more piscine than 

 cetacean ; or, we may say, the common vertebrate number of 

 limbs, in Mosasaurs as in Ichthyosaurs, has not been departed 

 from, as in some later and more modified forms of warm- 

 blooded marine Vertebrates. Are the pelvic and femoral 

 rudiments, which are concealed beneath the skin in modern 

 whales, the remnants of the ventral fins of the cold-blooded 

 marine air-breather? or are they remnants of the hind legs 

 of a warm-blooded, ancestral, shore-haunting quadruped ? 

 Such pleasant speculations as a solace to the work of acqui- 

 sition of positive facts may be condoned. 



The framework of the pectoral fin was first restored by 

 Prof. Marsh *, in the American Mosasauroid which he calls 

 Lestosaurus si'mus [Platecarjms , Cope) : the Lacertian type of 

 this framework is illustrated, in comparison with the Cetacean 

 and Enaliosaurian types, in the undercited work f. In the 

 large proportion of a Leiodon discovered by Prof. Snow of the 

 Kansas University, in the " yellow limestones along the 

 Hackbury Creek, in Yove County, Kansas," the framework 

 of the pectoral fin was found u lying underneath the ribs and 

 vertebras, with the bones in natural position." Fig. 13, PI. 

 VIII., is a reduced copy of the Professor's figure of this fin %. 

 Assuming the accuracy of the arrangement of the bones, the 

 first (innermost or radial) digit (i) is the longest, and includes 

 a metacarpal (m) and eleven phalanges j the second digit (u) 

 has the same number of phalanges and nearly the same length ; 

 the third (ill) has nine phalanges ; the fourth (iv) shows seven, 

 but one or two small terminal bones seem to be wanting ; the 

 fifth, outermost or ulnar, digit (v) has evidently been the 

 shortest, and retains but five phalanges. In the general shape of 



* American Journal of Science and Arts, vol. iii. p. 4, pi. x. (1872). 



t Quarterly Journal of the Geological Society, vol. xxxiv. p. 74S, 

 figs. 1-J (1878). 



] Transactions of the Kansas Academy of Sciences, vol. vi. p. 57, 

 (1878). This volume had not appeared at the date of the reading of my 

 second paper, " < hi the Affinities of the Mosasauridse," Juno 5, 1878, 



