14 HOOFS, NAILS AND CLAWS cuAi'. 



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

 that l:)irds with highly-variegated plumage are provided only 

 with tlie uropygial gland, while mammals with usually dull and 

 sindlar coloration have a great variety of skin glands. Scent 

 is no doubt a sense of liiglier importance in mammals than in 

 l)irds. Tlie su1)ject is one which will bear further study. 



Nails and Claws. — Except for tlie Cetacea (where rudi- 

 ments have been found in the foetus), the extremities of the 

 fingers and of the toes of mamuials 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 plialangeal horny 

 coverings. But the differences become extinguished as we pass 

 from these to reLited types. The nail^ of the little finger in Man 

 approaches the claw-like form ; and the hoofs of the Lama ;ire 

 almost claws in the sharpness of their extremities. On tlie 

 whole it may l)e said that claws and lioofs embrace the bone 

 which tliey cover, while nails lie only upon its dorsal surface. 

 The form of the distal phalanx which bears the luiil shows, 

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

 classification. AVhen those phalanges are clad with lioofs or 

 covered l)y a ludl they end in a rounded and fiattened termina- 

 tion. On the other hand, when they bear a claw they are them- 

 selves sliar])eucd at tlie 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 

 Ijelieved to be a characteristic of the group Marsupialia. Rudi- 

 ments of this structure have, liowever, l)een recently discovered 

 in tlie higher mammals, and, as Dr. Klaatsch ^ has remarked, all 

 researches into the '" history of the mammals culminate in the 

 question whether the placental manniials pass through a mar- 

 supial stage or not." We cannot, therefore, look upon the 

 marsupial pouch as a matter affecting only the ]\Iarsupials, 

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

 them and in the Mouotremata. 



^ " liber ^Iarsu]nalni(liincnti' ln-i Placentalicrn," J/r*;-/;//. Jalirh. xx. ]89r!, ]i. 

 27G. 



