492 0. C. Marsh—On Dinosaurian Reptiles. 
Further discoveries may in time solve the problem of the 
origin of all the Reptiles now called Dinosaurs, but the argu- 
ments 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 claim equal probability only with a 
similar suggestion recently made in regard to mammals. This 
subject of the origin of the Dinosaurs and the relation of their 
divisions to each other will be more fully treated by me 
elsewhere. 
A. classification of any series of extinct animals is of neces- 
sity, as I have previously said, merely a temporary convenience, 
like the book shelves in a library, for the arrangement of 
present knowledge. In view of this fact and of the very 
limited information we now have in regard to so many Dino- 
saurs known only from fragmentary remains, it will suffice for 
the present, or until further evidence is forthcoming, to still 
consider the Dinosauria as a subclass of the great group of 
Leptilia. 
Regarding, then, the Dinosaurs as a subclass of the Leptelia, 
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 vertebree joined to centra 
by suture; sacral vertebre united. Chevrons articulated 
intervertebrally. Cervical and thoracic ribs double-headed. 
Clavicles wanting. Ilium prolonged in front of the acetab- 
ulum; 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 caleaneum only, which together form the upper portion of 
ankle joint; reduction in number of digits begins with the 
fifth. ) 
Order THEROPODA (Beast foot). Carnivorous. 
Skull with external narial openings lateral; large antorbital 
vacuity ; brain case incompletely ossified; no pineal foramen. 
Premaxillaries with teeth ; no predentary bone; dentary with- 
out coronoid process; teeth with smooth compressed crowns, 
and crenulated edges. Vertebrz more or less cavernous; pos- 
terior trunk vertebree united by diplosphenal articulation. 
