418 THE VERTEBRATE SKELETON. 



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; simi- 

 lar 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 re- 

 presented 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. 



(/) Horny plates of a ridged or roughened character 

 occur upon the anterior portion of the palate, and of the man- 

 dibular symphysis in all three genera of recent Siren ia ; also 

 upon the toothless anterior portion of the palate in Ruminants. 



(g) The baleen of whales also belongs to the epidermal 

 exoskeleton. It consists of a number of flattened horny plates 

 arranged in a double series along the palate. The plates are 

 somewhat triangular in form and have their bases attached 

 to the palate at right angles to its long axis, while their apices 

 hang downwards into the mouth cavity. The outer edge 

 of each plate is hard arid smooth, while the inner edge and. 



