94 GENERAL ORNITHOLOGY part ii 



passages run through them to admit air to other parts of the body, 

 as under the skin and in various bones. Reproduction is oviparous ; 

 the eggs are very large, in consequence of their copious yolk and 

 white ; have a hard chalky shell, and are hatched outside the l)ody 

 of the parent. There ai-e always four limbs, of which the fore 

 or pectoral pair are strongly distinguished from the hind or pelvic 

 pair by being modified into wings, fitted for flying, if at all, by 

 means of feathers — not of skin as in the cases of such mammals, 

 reptiles, and fishes as can fly. The terminal part of the limb is 

 compressed and reduced, bearing never more than three digits, only 

 two of which ever have claws, and no claws being the rule. There 

 are not more than two separate carpals, or wrist-bones, in adult 

 recent birds (with very rare exceptions) ; nor any distinct inter- 

 clavicular bone. The clavicles are complete (with rare exceptions), 

 and coalesce to form a "wish-bone" or "merrythought." The 

 sternum, or breast-bone, is large, usually carinate, or keeled, and the 

 ribs are attached to its sides only ; it is developed from two to five 

 or more centres of ossification. The sacral vertebras proper have no 

 expanded ribs abutting against the ilia; the ilia, or haunch-bones, 

 are greatly prolonged forward ; the socket for the head of the 

 femur, or thigh-bone, is a ring, not a cup ; the ischia and ^jw/yes are 

 prolonged backward in parallel directions, and neither of these bones 

 ever unites with its fellow in a ventral symphysis (except in Struthio 

 and Rhea). The fibula, or outer bone of the leg, is incomplete 

 below, taking no part in the ankle-joint. The astragalus, or upper 

 bone of the tarsus, unites with the tiJna, or inner bone of the leg, 

 leaving the ankle-joint between itself and other tarsal bones, the 

 lower of which latter similarly unites with the bones of the instep, 

 or metatarsus. There are never more than four metatarsal bones, 

 and the same number of digits ; the first or inner metatarsal bone 

 is usually free, and incomplete above ; the other three anchylose 

 (fuse) together, and with distal tarsal bones, as already said, to form- 

 a compound tarso-metatarsus. Recent birds, at any rate, have a 

 certain saddle-shape of the ends of the bodies of some vertebras. 

 Such birds have also no teeth and no fleshy lips ; the jaws are 

 covered with horny or leathery integument, as the feet are also, 

 when not feathered. 



The Position of the Class Aves among other Vertebrates is 

 definite. Birds come in the scale of development next below the 

 Class Mammalia, and no close links between Birds and Mammals are 

 known ; the most bird-like known mammal, the duck-billed platypus 

 of Australia {Ornithorhynchus paradoxus), being several steps beyond 

 any known bird. Birds are the higher one of the two classes of 

 Sauropsida — the lower class, Beptdia, connecting with the Batrachians 

 (frogs, toads, newts, etc.), and so with the Fishes, Ichthyopsida. In 



