14 HOOFS, NAILS AND CLAWS chap. 



important part in their life, and it is perhaps worthy of note 

 tliat birds with highly-variegated plumage are provided only 

 with tlie 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 pihalangeal 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 c(_)ver, while nails lie only upon its dorsal surface. 

 The form of the distal x^halanx which bears the nail shows, 

 however, two kinds of modification which do not support such a 

 classification. AVhen those phalanges are clad with hoofs or 

 co\eied 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 mammals, and, as Dr. Klaatsch ^ has remarked, all 

 researches into the " history of the mammals culminate in the 

 question wliether 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 ^larsupials, 

 though it is true that this organ is at present functional only in 

 them and in the Monotremata. 



' " Uber Marsupialrudimi'nte bei Plaeentaliern,'' jl/ory/i. Jahrb. xx. ISfl.S, p. 

 276. 



