442 The American Naturalist. [May, 
He reached the result “that the Dinosauria has no existenceas 
a natural group of animals, but includes two distinct types of 
animal structure.” These two orders are called Ornithischia and 
Saurischia. 
ORNITHISCHIA. 
“Tn this order the ventral border of the pubic bone is divided so 
that one limb is directed backward parallel to the ischium, as . 
among birds, and the other limb is directed forward. Neither of 
these limbs of the pubis appears to form a median symphysis. a 
The ilium is prolonged in front of the acetabulum as a more or : 
less slender processor bar. The vertebræ are solid, and the skele- 
ton is not pneumatic. The basicranial structure is distinctive 
differing from that of crocodiles and lizards. The body and 
limbs are frequently covered with scutes, which many form a com- J 
plete shield or be reduced so as to be unrecognizable. The digits 
vary from three to five.” 
E a 
SAURISCHIA. 
“ In this order the pubis is directed forward from its symphysis 
with the ischium, and no posterior limb of the bone is developed. i 
Both pubis and ischium appear to meet by a median symphysis, 3 
so that the arrangement.and relation of the bones are lacertilian. e 
The anterior prolongation of the ilium has a vertical expansion. : 
The vertebre are more or less pneumatic or cavernous, and in 
the dorsal region the neural arch is commonly elevated. The 2 
` basicranial structure is sub-lacertilian. No armor has been found. 
The digits vary in number from three to five.” o 
In 1889 Marsh (24)admits four orders of Dinosauria: Sauropoda, 
Stegosauria, Ornithopoda, Theropoda ; Ceratosaurus, Hallopus, ae 
and Compsognathus being placed among the Theropoe oe 4 
Cope (25) admits, partially at least, Seeley’s classification, be 
keeps the order Dinosauria, which he divides in two n coe 
Saurischia and Orthopoda; the first with the inferior pe o 
elements directed downwards, the second with the pelvic elements a 
directed backwards. a 
Lydekker (26) divides the order Dinosauria in three sub 
Sauropoda, Theropoda, Ornithopoda. In the Ornithopoda ™ . 
includes the Stegosauria of Marsh. l : 
