(_c_) The =horns of Rhinoceroses=. These are conical structures composed of a solid mass of hardened epidermal cells growing from a cluster of long dermal papillae. From each papilla there grows a fibre which resembles a thick hair, and cementing the whole together are cells which grow from the interspaces between the papillae. These fibres differ from true hairs in not being developed in pits in the dermis. Rhinoceros horns may be either one or two in number, and are borne on the fronto-nasal region of the skull. They vary much in length, the longest recorded having the enormous length of fifty-seven inches. (_d_) _Nails_, _hoofs_ and =claws=. In almost all mammals except the Cetacea, these are found terminating the digits of both limbs. =Nails= are more or less flattened structures, =claws= are pointed and somewhat curved. In most mammals the nails tend to surround the ends of the digits much more than they do in man. Sometimes the nail of one digit differs from that of all the others; thus the second digit of the pes in the Hyracoidea and Lemuroidea is terminated by a long claw, the other digits having flat nails. In the Felidae the claws are retractile, the ungual phalanx with claw attached folding back when the animal is at rest into a sheath, above, or by the side of the middle phalanx. In the Sloths and Bats enormously developed claws occur, forming hooks by which the animals suspend themselves. In _Notoryctes_ the third and fourth digits of the manus bear claws of great size; similar claws occur in _Chrysochloris_, being correlated in each case with fossorial habits. The nail at its maximum development entirely surrounds the terminal phalanx of the digit to which it is attached, and is then called a =hoof=. Hoofs are specially characteristic of the Ungulata. (_e_) =Spurs= and =beaks= are structures which are hardly represented among mammals, while so characteristic of birds. They are however both found in the Monotremata. In both _Echidna_ and _Ornithorhynchus_ the male has a peculiar hollow horny spur borne on a sesamoid bone articulated to the tibia. The jaws in _Ornithorhynchus_ are cased in horny beaks similar to those of birds, and are provided with horny pads which act as teeth. (_f_) =Horny plates= of a ridged or roughened character occur upon the anterior portion of the palate, and of the mandibular symphysis in all three genera of recent Sirenia; also upon the toothless anterior portion of the palate in Ruminants.