Paleontology. 179 
plantigrade into a highly digitigrade type—the 
curved lines of connexion serving to indicate the 
homologous bones (Figs. 76, 77). 
I will now proceed to detail the history of mammalian 
limbs, as this has been recorded for us in fossil remains. 
The most generalized or primitive types of limb 
hitherto discovered in any vertebrated animal above 
Fic. 78.—A, posterior limb of Laplanodon discus. (Aftcr Marsh.) F, 
thigh-bone; Ito VI, undifferentiated bones of the leg and foot. B, 
anterior limb of Chelydra serpentina. (Afler Gegenbaur.) U and R, 
bones of the fore-arm; I to V, fully differentiated bones of the hand, 
following those of the wrist. 
the class of fishes, are those which are met with in 
some of the extinct aquatic reptiles. Here, for 
instance, is a diagram of the left hind limb of 
Baptanodon discus (Fig. 78). It has six rows of little 
symmetrical bones springing from a leg-like origin. 
N2 . 
