444 The American Naturalist. ' [May, 
2. The Dinosauria form an order of reptiles, containing three 
suborders: Sauropoda, Ornithopoda, Theropoda (Lydekker); 
Sauropoda, Orthopoda, Theropoda (Zittel). 
3. The Dinosauria form an order of reptiles, containing two 
suborders : Saurischia, Orthopoda (Cope). 
B. The Dinosauria are not a Natural Group—The reptiles 
generally called Dinosauria belong to two distinct orders: 
Ornithischia and Saurischia (Seeley). ; 
The first question to decide is, Do the Dinosauria represent 
a natural group or not? To examine this we will proceed to 
study a member of each of the three groups, Sauropoda, 
Orthopoda, and Theropoda, and compare these members 
among themselves. Of the Orthopoda especially we will take 
as a type Iguanodon, the structure of which is best known 
through the different publications of Dollo in the Bull. Musée 
Royal His. Nat. de Belgique; of the Sauropoda we will 
take Diplodocus, described by Marsh; and of the Theropoda, 
Ceratosaurus, also made known by Marsh. We begin with 
the skull, then treat the vertebræ, the shoulder girdle, the 
pelvis, the fore and hind limbs, the abdominal ossicles, and 
the dermal ossification so far as necessary. 
I. THE SKULL. l 
/guanodon.—Al\ that I have to say about Iguanodon is based 
on the careful descriptions of Dollo (32). ; 
1. The brain-case is completely ossified; a very strong xÍ 
sphenoid being present. i 
2. The premaxillaries are separate, and there is a strong P? 
cess extending between the nasals and mandibles, excluding ~ oo 
maxillaries from the nasal opening. 
3- No epipterygoid (columella). Ses 
4. The jugals are Pe toa nie process of the maxilares; 
they are not placed in the same level with the alveolar border, 
but a considerable distance outside of it. They do pa E 
theend of the dental series. They are in connection with da 
lachrymals, postfrontals, quadratojugals, and maxillaries. ; 
bound the orbits inferiorly, and also somewhat posteriorly- 
