2 BULLETIN 50, UJSriTED STATES NATIONAL MUSEUM. 



much more of the hidden irij^steries of creation, and adds proportion- 

 atel}' to the sum of human knowledge/ 



Birds constitute a far more homogeneous group than any other class 

 of the animal kingdom, and their classification is therefore a matter of 

 unusual difficulty. According to universal agreement, they constitute 

 a single class, Aves, whose characters and subdivisions (according to 

 our present knowledge) are as follows: 



CLASS AVES.— BIRDS. 



Birds iiXQ feathered veirtebrate animals. ^ 



The more recent investigations of comparative anatomists have grad- 

 uall}' eliminated the supposed exclusive characters of birds, as a Class 

 of the Animal Kingdom, until only the single one mentioned above, 

 the possession of feathers, remains. No other structural character is 

 possessed by them which is not shared either bj^ the Class Reptilia or 

 Class Mammalia; but "no bird is without feathers, and no animal is 

 invested with feathers except the birds. " ' Indeed, so closely are birds 

 related to reptiles that in all other structural characters whereby they 

 differ from mammals they agree with reptiles; and notwithstanding 

 their extreme dissimilarity in appearance and habits they are essentially 

 "an extremely modified and aberrant Reptilian type."* 



Birds differ from all Mammals in the following characters: 



(1) Possession of feathers. 



(2) Absence of milk glands. 



(3) Single occipital condyle. 



'The most complete review of the history of ornithology is that forming the 

 introduction to Professor Newton's Dictionary of Birds (London: Adam and Charles 

 Black, 1893-1896; The Macmillan Company, New York). 



■^ The following diagnosis of the class is given by Gadow, in Proc. Zool. Soc. Lond. 

 1892, p. 236: 



Oviparous, warm-blooded, amniotic vertebrates which have their anterior extrem- 

 ities transformed into wings. Metacarpus and fingers carrying feathers or quilla. 

 AVith an intertarsal joint. Not more than four toes, of which the first is the hallux. 



This later (A Classification of Vertebrata recent and extinct, 1898, p. 30) amended 

 as follows: 



Warm-blooded, oviparous, Amniota, AUantoidae. Occipital condyle single. Quad- 

 rate movable. Anterior extremities transformed into wings. Covered with feath- 

 ers. With intertarsal joint. Not more than four toes, of which the first is the 

 hallux. 



■' Stejneger, Standard Natural History, i\-, 1885, p. 1. Dr. Stejneger's most excel- 

 lent article (pp. 1-20 of the work cited) should be consulted by those who desire 

 more detailed information on the subject. 



■'Newton, in the article on Ornithology in the Encyolopredia Britannica; also 

 HtrxLEY, Lectures on the Elements of Comparative Anatomy, p. 69; Caeus, Hand- 

 buch der Zoologie, p. 192. 



