14 HOOFS, NAILS AND CLAWS CHAP. 
important part in their life, and it is perhaps worthy of note 
that birds with highly-variegated plumage are provided only 
with the uropygial gland, while mammals with usually dull and 
similar coloration have a great variety of skin glands. 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 rudi- 
ments 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 termina- 
tion. On the other hand, when they bear a claw they are them- 
selves 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, Rudi- 
ments of this structure have, however, been recently discovered 
in the higher inamimals, and, 2s Dr. Klaatsch’ has remarked, all 
researches into the “history of the mammals culminate in the 
question whether the placental mammals pass through a mar- 
supial 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, 
1 Uber Marsupialrudimente bei Placentaliern,” Morph. Jahrb. xx. 1893, p. 
276, 
