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 Bapsa- 
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 
