assume the function of a hand; and in a few of the kangaroo tribe, such as the opossum, the hinder extremities are provided with a thumb capable of being brought into apposition with the other toes. The hand of bats, which serves these animals not merely for flying, but also supplies the place of eyes and ears, enabling them to avoid objects during the obscurity of night, is furnished with a sense of touch extremely delicate. It is in man and the monkey tribe only that we meet with a true and perfect hand. In the latter, both the fore and hind extremities are constructed upon the same general principle as the hand of man; and hence these animals have been styled four-handed (_Quadrumana_). “In man the upper members are disposed in a manner the most favourable for the exercise of their functions as organs of prehension and of touch: the fingers are long and flexible; they have all, with the occasional exception of the ring-finger, separate movements, which other animals (even those provided with hands) have not; the thumb, which is apposable to these latter, is longer in proportion to the fingers than in monkeys, and consequently can more readily apply itself to the extremity of their palm or surface, and better seize small objects. The nails, which are large and flat, cover the back part only of the extremity of the fingers, so as to furnish a support to the organs of touch without in the least depriving them of their delicacy. The entire hand can execute rotatory movements the most extended; and to the arm which supports it is afforded a solid attachment by means of a large shoulder or blade-bone and a long collar-bone.” The statement of Milne Edwards,[3] then, does not appear far removed from the truth, “That the faculties of the _Mammalia_ are the more elevated in proportion as their members are the better constructed for prehension and for touch.” USE OF A KNOWLEDGE OF THE FORM OF APPENDAGES TO TRUNKS IN GEOLOGY.