Scent is no doubt a sense of higher importance in mammals than in birds. The subject is one which will bear further study. NAILS AND CLAWS.--Except for the Cetacea (where rudiments have been found in the foetus), the extremities of the fingers and of the toes of mammals are covered by, or encased in, horny epidermic plates, known as nails, claws, and hoofs. The variety in the shape and development of these corneous sheaths to the digits is highly characteristic of mammals as opposed to lower Vertebrates. If we take extreme cases, such as the nail of the thumb in Man, the hoof of a Horse, and the claw of a Cat, it is easy to distinguish the three kinds of phalangeal horny coverings. But the differences become extinguished as we pass from these to related types. The nail of the little finger in Man approaches the claw-like form; and the hoofs of the Lama are almost claws in the sharpness of their extremities. On the whole it may be said that claws and hoofs embrace the bone which they cover, while nails lie only upon its dorsal surface. The form of the distal phalanx which bears the nail shows, however, two kinds of modification which do not support such a classification. When those phalanges are clad with hoofs or covered by a nail they end in a rounded and flattened termination. On the other hand, when they bear a claw they are themselves sharpened at the extremity and often grooved above. THE MARSUPIUM.--It may appear to be unnecessary at this juncture to speak of the marsupial pouch, which is so usually believed to be a characteristic of the group Marsupialia. Rudiments of this structure have, however, been recently discovered in the higher mammals, and, as Dr. Klaatsch[6] has remarked, all researches into the "history of the mammals culminate in the question whether the placental mammals pass through a marsupial stage or not." We cannot, therefore, look upon the marsupial pouch as a matter affecting only the Marsupials, though it is true that this organ is at present functional only in them and in the Monotremata. {15} [Illustration] FIG. 3.--_Echidna hystrix._ A, Lower surface of brooding female; B; dissection showing a dorsal view of the pouch and mammary glands; ++, the two tufts of hair in the lateral folds of the mammary pouch from which the secretion flows, _b.m_, Pouch; _cl_, cloaca; _g.m_, groups of mammary glands. (From Wiedersheim's _Comparative Anatomy_, after W. Haacke.) In the Marsupials the pouch shelters the young, which are born in an exceedingly imperfect state, minute, nude, and blind, with a "larval" mouth fitted only to grasp in a permanent fashion the teat, upon which they are carefully fixed by the parent. But even later the pouch is made use of as a temporary harbour of refuge: from the pouch of female Kangaroos at the Zoological Gardens may frequently be observed to protrude the tail and hind-legs of a young Kangaroo as big as a Cat, and perfectly well able to take care of itself.