Fig. 5.—Part of the Vertebral Column of the Fossil Batrachian Eryops megacephalus, one fourth natural size, showing segmentation, the left side.
as those of the horse, the pig, and of the ruminants, the ends of the toes are applied to the ground, and are covered with larger hoofs, which surround the toe, and the cushion is nearly or quite dispensed with. These animals are especially distinguished by the fact that their metapodial keels extend entirely round the end of the bone, dividing the front, as well as the end and back, into two parts. This structure would seem to be a result of the greater force of the impact resulting from use of the legs, experienced by the end and front of the bone, which receives the blows.
I cite one more instance of the effect of use on the structure of the skeleton, and this time from the vertebral column. In a certain order of Batrachia from the Permian formation (the Rhachitomi), the bodies of the vertebræ are curiously segmented. Instead of a solid slightly modified from a cylinder, as in most vertebrates, we have three pieces. One of these is of nearly the form of a segment cut from an apple or orange having a crescentic outline and wedge-shaped section. The sharp edge is concave and is directed upward. On one side of each of the horns of this crescent a rhombic piece is applied, which, widening upward, supports the separate arch of the vertebra. These three pieces when together leave a central vacancy, which becomes, when all the vertebræ are placed end to end, a canal or tube. This was occupied by what is known as the chorda dorsalis, which is the central axis of the body of the simplest vertebrates, and is present in the early embryonic stages of all. In the growth of a reptile or mammal this flexible chorda is replaced by the bony vertebral bodies. The osseous material appears in the membranous sheath which covers it, and, gradually encroaching on it, first cuts it into segments and then fills it entirely. In the rhachitomous batrachia this process is not completed, as the chorda remains more or
