NOTES 



471 



38 The posterior pair of limbs is sometimes represented by a pair of 

 small bones; and the boas and pythons show traces of external hind 

 limbs. 



39 The plastron is formed partly of dermal and partly of endoskeletal 

 pieces. 



40 Knees always bend, forward, and heels always bend backward. 



41 We cannot claim that this airy skeleton is necessary for flight. The 

 bones of the bat are free from air, yet it is able to keep longer on the 

 wing than the sparrow. The common fowl has a hollow humerus ; while 

 some birds of long flight, as the snipe and curlew, have airless bones. 



42 Hopping is characteristic of and confined to the perchers ; but many 

 of them, as the meadow lark, blackbird, and crow, walk. 



43 This order is artificial. But it is better to retain it until ornitholo- 

 gists agree upon some natural arrangement. 



44 The whales are hairy during foetal life only. 



45 The manatee has 6 ; Hoffmann's sloth 6 ; and two species of three- 

 toed sloth have respectively 8 and 9. 



46 As in the whale, porpoise, seal, and mole. Teeth are wanting in the 

 whalebone whales, ant-eaters, manis, and echidna. 



47 The monotremes resemble marsupials in having marsupial bones, but 

 have no pouch. They differ from all other mammals in having no distinct 

 nipples. 



48 The pouch is wanting in some opossums and the dasyurus. 



49 The extinct horse {Hipparion) had three toes, two small hoofs dan- 

 gling behind. The foot of the horse is of wonderful structure. The bones 

 are constructed and placed with a view to speed, lightness, and strength, 

 and bound together by ligaments of marvelous tenacity. There are elastic 

 pads and cartilages to prevent jarring ; and all the parts are covered by a 

 living membrane which is exquisitely sensitive, and endows the foot with 

 the sense of touch, without which the animal could not be sure-footed. 

 The hoof itself is made of parallel fibers, each a tube composed of thousands 

 of minute cells, the tubular form giving strength. There are three parts, 

 "wall," "sole," and "frog" the triangular, elastic piece in the middle, 

 which acts as a cushion to prevent concussion and also slipping. 



60 The fore feet of the tapir have four toes, but one does not touch the 

 ground. 



51 The camel and llama are exceptional, having two upper incisors and 

 canines, are not strictly cloven-footed, having pads rather than hoofs, and 

 are hornless. 



52 For the best account of the elephant, see Tennant's " Ceylon " or 

 Brehm's "Thierleben." 



53 The hyena alone of the carnivores has only four toes on all the limbs, 

 and the dog has four hind toes. 



64 The old term Quadrumana is rejected, because 'it misleads, for apes, 



