182 DINOCERATA. 



The most generalized limbs in any vertebrates, above the class of 

 fishes, are seen in some of the extinct aquatic reptiles, especially in forms 

 allied to Ichthyosaurus. Here, as the author has shown, i we may find in 

 one group, 



(1.) Each limb a simple fin, or paddle, 



(2.) Fore and hind limbs identical in structure, 



(3.) Axis of limb through intermedial bone and third digit, 



(4.) Single bone (humerus or femur) in propodial, or first, segment, 



(5.) Three bones, including intermedial, in epipodial, or second, 



segment, 



(6.) Mesopodial bones (carpals or tarsals) circular disks, 



(7.) Number of digits six or more, 



(8.) Metapodial bones and phalanges circular disks, 



(9.) Phalanges, very numerous. 



This is a primitive aquatic limb, flexible, but without joints, and 

 adapted to swimming only. An example of such a limb is seen in 

 figure 148, below. 



For progression both in water and on wet ground, an essential 

 modification of such a limb would be required, and a type seen in some 

 of the living reptiles would gradually be developed. This limb would 

 be jointed at two points, and have five digits, with the axis through the 

 middle one. The foot in this limb would be very similar to the 

 generalized foot of the primitive Mammal, and may here be taken as its 

 representative. An example of such a foot is shown in figure 149. 



In the true Ungulate Mammals, the modifications of the feet have 

 undoubtedly taken place very nearly in the following manner : 



(1.) The primitive Ungulates {Protungulata) must have had 

 plantigrade, pentadactyl, feet, with the carpals and tarsals not interlocking, 

 either with the metapodial bones, or with their own adjoining series. 

 This Avould give a weak foot, adapted especially to progression in soft 



' Limbs of Sauraiiodon, etc., American Journal of Science, 1880. 



