238 DINOSAURS OF NOKTH AMERICA. 



The remarkable discoveries in North America, however, have changed 

 the whole subject, aud in place of fragmentary specimens many entire 

 skeletons of dinosaurian reptiles have been brought to light, and thus 

 definite information has replaced uncertainty and rendered a compre- 

 hensive classification for the first time possible. 



The system of classification first proposed by the writer in 1881 has 

 been very generally approved, but a few modifications have been sug- 

 gested by others that will doubtless be adopted. This will hardly be 

 the case with several radical changes recently advocated, based mainly 

 upon certain theories of the origin of dinosaurs. At present these 

 theories are not supported by a sufficient number of facts to entitle 

 them to the serious consideration of those who have made a careful 

 study of these reptiles, especially the wonderful variety of forms recently 

 made known from America. 



Further discoveries may in time solve the problem of the origin of 

 all the reptiles now called dinosaurs, but the arguments hitherto 

 advanced against their being a natural group are far from conclusive. 

 The idea that the Dinosauria belong to two or more distinct groups, 

 each of independent origin, can at present only claim equal probability 

 with a similar suggestion recently made in regard to mammals. This 

 subject of the origin of the dinosaurs and the relation of their divi- 

 sions to each other will be more fully treated by the writer elsewhere. 



A classification of any series of extinct animals is of necessity, as the 

 writer has previously said, merely a temporary convenience, like the 

 bookshelves in a library, for the arrangement of present knowledge. 

 In view of this fact and of the very limited information in regard to so 

 many dinosaurs known only from fragmentary remains, it will suffice 

 for the present, or until further evidence is forthcoming, to still con- 

 sider the Dinosauria as a subclass of the great group of Reptilia. 



Regarding, then, the dinosaurs as a subclass of the Reptilia, the forms 

 best known at present may be classified as follows: 



Subclass Dinosauria Owen. 



Premaxillary bones separate; upper and lower temporal arches; no 

 teeth on palate; rami of lower jaw united in front by cartilage only. 

 Neural arches of vertebne joined to centra by suture; cervical and 

 thoracic ribs double headed; ribs without uncinate processes; sacral 

 vertebra} united; caudal vertebra; numerous; chevrons articulated 

 intervertebrally. Scapula elongate; no precoracoid; clavicles wanting. 

 Ilium prolonged in front of the acetabulum; acetabulum formed in part 

 by pubis; ischia meet distally on median line. Fore and hind limbs 

 present, the latter ambulatory, and larger than those in front; head of 

 femur at right angles to condyles; tibia with procnemial crest; fibula 

 complete; first row of tarsals composed of astragalus and calcaneum 

 only, which together form the upper portion of ankle joint; reduction 

 in number of digits begins with the fifth. 



