6. The number of palm-bones (metapodal) and toes deserves special notice. In fishes, and in some extinct swimming reptiles, these are or were very numerous, but in the earliest land-animals they became five. This is the number now in nearly all reptiles, and in all the more generalized mammals. It may be called the normal number for a walking animal. In very many mammals, such, for example, as the dog family, they are reduced to four, though the fifth often remains as a useless, rudimentary splint and dew-claw (Fig. 6), thus showing the process of dwindling in the ancestry. In hoofed animals the process of gradual diminution is shown even in existing forms, and still better in extinct forms. Confining ourselves, now, only to existing forms, in the elephant there are five palm-bones and toes, and in the hippopotamus there are four, all functional. In the hog (Fig. 7) there are still four, but two are behind the others and much smaller, and do not touch the ground--are not functional unless in soft ground. In the cow, deer, etc., the palm-bones are reduced to two, and these are consolidated into _one_ (canon-bone), and the toes are reduced to two efficient and two useless rudiments. In the sheep and the goat (Fig. 8) these useless rudiments are dropped, and there are two only. Finally, in the horse (Fig. 9), the _toes_ are reduced to one, although the palm-bones are still three, two of them, however, being reduced to rudimentary splints. How is it with birds? Have these also palm-bones and fingers? Yes, in birds (Fig. 11) there are three palm-bones and three fingers (the fourth and fifth being wanting); one of them--the thumb--is free, and sometimes carries a claw. In the earliest known and most reptilian bird, the archæopteryx (Fig. 12), all the three fingers are free, have the full number of joints, and all of them carry claws. In the embryo of living birds the fingers are all free, as in the archæopteryx. [Illustration: FIG. 19.--Restoration of Rhamphorhynchus phyllurus (after Marsh). One-seventh natural size.]