DEFINITION OF BIUIJS. 61 



are any living members of the class of Birds from any known Reptiles, th(.' characters of the 

 t\V(j groups converge in geologic history so closely, that the presence lA feathers in the former 

 class, and their absence from the latter, is one of the most pcjsitive differences we have found. 

 The oldest known birds are from the Jurassic rocks of Europe, and the Cretaceous beds of 

 North America. These, birds had teetli, and various other strong peculiarities of structure, 

 which no living members of the class have retained. 



AVES, or the Class of Birds, nuiy be distinguished from other Saaropsida, for all that 

 is known to tlie c(jntrary, by tlie following sum of characters: Tlie body is covered with 

 feathei's, a kind of skin-outgrowth no other animals possess. The blood is \uA ; the circu- 

 lation is completely double ; the heart is perfectly four-chambered ; there is but one (the light) 

 aortic arch, and only one pulmonary artery springs from the heart ; the aortic and the pulmo- 

 nary artery have each three semilunar valves. The lungs are fixed and moulded to the ca"S'ity 

 of the chest, and some of the air-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 the copious yolk and white ; have a hard chalky shell, and are liatched 

 outside the body of the parent. There are always four limbs, of wliich tlie fore or pectoral 

 pair are strongly distinguished from the hiud 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 tlie rule. There are not more than two separate carpals, or wrist-bones, in adult recent 

 birds (with very rare exceptions) : nor any distinct interclavicular bone. The clavicles are 

 complete (with rare exceptions), and coalesce to form a ''wish-bone" (jr " merry-thvught." 

 The steniiim, or breast-bone, is large, usually carinate, or keeled, and tlie ribs are attached to 

 its sides <->nly ; it is developed from two to five or more centres of fissificatiou. Tlio sacred ver- 

 tebra; proper liave no expanded ribs abutting against the ilia ; the ilia, or haunch-bones, are 

 greatly prolonged forward ; the socket for the head of tlio femiir,ox thigh-bone^is a ring, not a 

 cup ; the ischia and puhes are prolonged backward in parallel directions, and neither (jf these 

 bones ever unites with its feUow in a ventral symphysis (except in Strutliio and Rhea). The 

 fibula, or outer bone of the leg, is incomplete below, taking no part in the ankle-joint. The 

 astrar/alas, or upper bone of the tarsus, unites with the fibia,ov inner bone of the leg, leaving 

 the anlile-joint betM'een itself and other tarsal bones, the lower of which latter similarly unites 

 wdth 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 incom- 

 plete 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 vertebrse. 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 (jf development next below the Cluss 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 liigher one of the two classes a's Sauropsida — tlie lower class, Reptilia, connecting with 

 the Batrachians (frogs, toads, newts, etc.) and so with the Fishes, Ichthyopsida. In this Verte- 

 l)rate series, Birds constitute what is called a highly specialized group ; that is to say, a very par- 

 ticular oflF-shoot, or, more literally, a side-issue, of the Vertebrate genealogical tree, which in 

 tlie present geological era has become developed into very numerous (about 10,000) species, 



