DEPARTMENT OF GEOLOGY 
25 
than the Icthyosaur. Some forms had a very long neck. The 
body was short and armed with stout, paddle-like legs which 
were used in swimming. In the long-necked Plesiosaurs the 
head was broad and snake-like; in the short-necked types it 
ended in a slender snout similar to that of the Icthyosaurs. A 
model of one of the long-necked types exhibited on the floor 
of Hall 36 will give some idea of the general proportions of the 
animal. 
Pterodactyls, common to Europe and known also from the 
American Jurassic formations are represented by incomplete 
specimens. More will be seen of them in the next period. 
Cretaceous Period . — Vertebrate fossils of the Cretaceous 
period are represented by specimens of Dinosaurs, Plesiosaurs, 
Pterodactyls and Fishes. They are of different types from 
those of the Jurassic. In addition there appeared a new type 
of marine reptiles — the Mosasaurs. 
Cretaceous Dinosaurs are illustrated by a fine skull with a 
partial skeleton of Triceratops and a restoration of the skeleton 
of Hadrosaurus (Hall 36). Triceratops was a unique armored 
land reptile of gigantic proportions. It is very different in 
structure from the Jurassic Dinosaurs though commonly included 
in that order. The head was very large in proportion to the 
size of the body. It was armed with three horns as indicated 
by the bony horn cores on the skull. There was also a wide 
bony frill, covered in life by a dermal shield, which projected 
backward from the skull and served to protect the neck and 
shoulders of the animal from the attacks of its homed enemies. 
The specimen shown is one of the largest of its kind. It was 
collected in eastern Montana by a special Museum expedition in 
1904. Parts of the skeleton of the same individual will be found 
in a wall case in the next hall. 
illustrated by a restoration, is a large biped rep- 
tile found in the Cretaceous deposits of New Jersey. The ani- 
mal walked entirely upon its hind legs, only using the smaller 
forward pair when resting or feeding. 
Mosasaurs were a new type of marine reptile which appeared 
with the Cretaceous period. They were short-necked, round- 
bodied, long-tailed animals, with pointed snouts. The legs were 
paddle-like and adapted to swimming. The tail probably ter- 
minated in a narrow fin which was also used as an organ of 
propulsion. The skeletons of these animals are found in large 
