BULLETIN 50, UNITED STATES NATIONAL MUSEUM. 



much more of th(> bidden mysteries of creation, and adds proportion- 

 ately to the sum of human knowledge.^ 



Birds constitute a far mor(> homoo-eneous group than any other class 

 of the animal kingdom, and their classitication i.s therefore a matter of 

 unusual ditticulty. According to universal agreement, they constitute 

 a single class. Ayes, whose characters and subdivisions (according to 

 our iM'i'-iMil i<ii'. >\vl(Hlg(») are as follows: 



CLASS AVES.— BIRDS. 



Birds AYd feathered vertehrate animals. '' 



The more recent investigations of comparative anatomists have grad- 

 ually eliminated th(> supposed exclusive characters of birds, as a Glass 

 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 })y the Class Reptilia or 

 Cla.ss Manuualia; but "no bird is without feathers, and no animal is 

 invested with feathers except the birds. ""^ Indeed, so closeh^ are birds 

 related to reptiles that in all other structural characters whereby they 

 ditier from mammals the}" agree with reptiles; and notwithstanding 

 their extreme dissimilarity in appearance and habits thc}^ are essentially 

 '•an extremely modified and aberrant Reptilian type.''* 



Birds differ from all Mammals in the following characters: 



(1) Possession of feathers. 



(2) A])sence 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, 1893-189(); The MacmUlan Company, New York). 



-The following diagnosis of the class is given by G.vdow, in Proi-. Zool. Soc. Lond. 

 1S92, p. 2m: 



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

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

 With an intcrtar.sal joint. Not more than four toes, of which the first is the hallux. 



This later (A Classification of Yertebrata recent and extinct, bSHS, p. 30) amended 

 as follows: 



Warm-blooded, oviparous, Amniota, AUantoidie. 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. 



•'Stejxeger, Standard Natural History, iv, 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. 



^Newtox, in the article on Ornithology in the Encyclo])fedia Britannica; also 

 llrxLEV, Lectures on the Elements of Comparative Anatomy, ]>. (>9; Carus, Hand- 

 l»uch der Zoologie, p. 192. 



