Paleontology. 181 



fishes. Obviously it is a type suited only to aquatic 

 life. Consequently, when aquatic Vertebrata began to 

 become terrestrial, the type would have needed modi- 

 fication in order to serve for terrestrial locomotion. In 

 particular, it would have needed to gain in consolida- 

 tion and in firmness, which means that it would have 

 needed also to become jointed. Accordingly, we find 

 that this archaic type gave place in land -reptiles to 

 the exigencies of these requirements. Here for ex- 

 ample is a diagram, copied from Gegenbaur, of the right 

 fore-foot of Chelydra serpentina (Fig. 78). As com- 

 pared with the homologous limb of its purely aquatic 

 predecessor, there is to be noticed the disappearance 

 of one of the six rows of small bones, a confluence of 

 some of the remainder in the other five rows, a dupli- 

 cation of the arm-bone into a radius and ulna, in 

 order to admit of jointed rotation of the hand, and a 

 general disposition of the small bones below these 

 arm-bones, which clearly foreshadows the joint of the 

 wrist. Indeed, in this fore-foot of Chelydra, a child 

 could trace all the principal homologies of the mam- 

 malian counterpart, growing, like the next stage in a 

 dissolving view, out of the primitive paddle of Bapta- 

 nodon namely, first the radius and ulna, next the 

 carpals, then the meta-carpals, and, lastly, the three 

 phalanges in each of the five digits. 



Such a type of foot no doubt admirably meets the 

 requirements of slow reptilian locomotion over swampy 

 ground. But for anything like rapid locomotion over 

 hard and uneven ground, greater modifications would 

 be needed. Such modifications, however, need not 

 be other in kind : it is enough that they should con- 

 tinue in the same line of advance, so as to reach a 



