Mammals to Aquatic Life. 171 



With this the climax is reached in the development of the 

 flipper in existing mammals ; the modifications of the skeleton 

 become so important that finally the idea of phalanges com- 

 pletely disappears. Not that the transformation-process stops 

 here, however; the changes may proceed yet further, and analogy 

 will show how we have to imagine that this further trans- 

 formation will take place in the distant future. For we find 

 that in earlier periods of the earth's history there took place the 

 same process of the formation of flippers from the fore limbs 

 of terrestrial animals, namely in the case of the Plesiosauri 

 and Ichthyosauri ; while in the latter it reached a much higher 

 degree of development than has yet been attained in existing 

 whales. All that we know about the position of the two 

 groups is that they are not to be regarded as directly connected 

 with one another *, but that both must have sprung from 

 land-reptiles. The latter assertion is opposed to the views of 

 Gegenbaur f, who, on the ground of the resemblance of the 

 extremities, placed them near the fishes; in this he has 

 been recently upheld by D'Arcy W. Thompson J. Later 

 discoveries §, however, point with certainty to the conclusion 

 that this resemblance is merely due to convergence and that 

 the ancestors of both groups were land-reptiles. The flipper 

 of the Plesiosauri was the less differentiated of the two ; it 

 stood in relation to that of the Ichthyosauri as the flipper of 

 the whalebone whales does to that of the Odontoceti. In the 

 oldest Plesiosaurs the hyperphalangy was still very limited, 

 the separate bones of the hand and the forearm having as 

 yet undergone very little differentiation. I find this in the 

 impression of an as yet undescribed skeleton of a Mesosaurus 

 from the Karroo formation || (a Plesiosaurus therefore), the 

 hand of which has undergone very little differentiation ; its 

 five distal carpals bearing five long metacarpals, to which are 

 affixed two, three, four, five, and four phalanges. The pro- 

 cess of retarded ossification makes itself here already percep- 

 tible, for the phalanges carry double epiphyses. But even the 

 most highly differentiated Plesiosaurus-ti'ippers still show a 



* Zittel, ' Handbueh der Palfiontologie,' p. 478. 



t Gegenbaur, " Ueber das Gliedmassenskelet der Enaliosauren," 

 Jenaische Zeitscbrift, 1870, pp. 340 et seq. 



% D'Arcy W. Thompson, " On tbe Hind Limb of Ichthyosaurus and on 

 tbe Morphology of Vertebrate Limbs,'' Journ. Anat. & Pbys. vol. xx.,1886. 



§ H. G. Seeley, " On Neusticosaurus pusillus (Fraas), an Amphibious 

 Reptile having affinities with the Terrestrial Nothosauria and with the 

 Marine PlesiosauriaJ' Quart. Journ. Geol. Soc. Lond. vol. xxxviii., 1S82. 

 Cf. also Fraas, Baur, Zittel, toe. tit. 



|| Preserved in the Natural-History Museum, London ; Mr. Smith 

 Woodward was kind enough to draw my attention to the specimen. 



12* 



