166 DINOSAURS OF NORTH AMERICA. 



and the neck long, with its vertebra; opisthoccelian and lightened by 

 inner cavities, thus allowing free motion. 



The limb bones of all were solid, and the feet plantigrade, with five 

 toes on each. The tail was especially long and massive. The general 

 form and proportions of these reptiles are indicated in PI. XLII, which 

 represents the skeleton of a species of Brontosaurus, one of the typical 

 genera of the Atlantosauridse. 



A TLANTOSA VRIDM 

 ATLANTOSAURUS. 



The present family was named by the writer in 1877, the type genus 

 being Atlantosaurus. The type specimen of the first species described, 

 Atlantosaurus montanus, is the sacrum represented in fig. 1, PI. XVII, 

 which shows characteristic features of the sacrum of the entire group 

 Sanropoda, and thus distinguishes it from that of the other known 

 dinosaurs. A second and larger species, Atlantosaurus immanis, was 

 described by the writer in the following year, and on PI. XVI are repre- 

 sented two pelvic bones and a femur, which belong to the type specimen, 

 and give an idea of its gigantic size. The femur is over 6 feet long, 

 and this, with other portions of the skeleton, indicates an animal about 

 70 or 80 feet in length. The pubis and ischium, represented in posi- 

 tion in fig. 1, are especially characteristic of the family, as will be 

 seen by comparing them with the corresponding parts of other allied 

 genera, as shown in PI. XXXVI. 



At the same locality where these remains were found, portions of a 

 skull were discovered, one of which is figured on PI. XV. This speci- 

 men, which is the posterior part of the skull, is of much interest, and 

 shows characters which separate it from all other corresponding remains 

 of dinosaurs. The most marked feature is a distinct pituitary canal 

 leading from the brain cavity down through the base of the skull, as 

 shown in fig. 2 of PI. XV. This canal appears to be a marked char- 

 acter of the family Atlantosaurkhe. Other points of interest in these 

 remains will be discussed later in the present paper. 



APATOSATJRUS. 



Another genus of the present family is Apatosaurus, also described 

 by the writer in 1877, and from the same geological horizon in Colo- 

 rado. The sacrum represented in fig. 2, PI. XVII, may be regarded as 

 the type specimen. It has the same general features as the sacrum of 

 Atlantosaurus, shown on that plate, but it has only three coossified 

 vertebrae instead of four. 



THE SACRAL CAVITY. 



The neural canal iu this sacrum, and indeed in all the sacra of the 

 Sauropoda, is much enlarged, being especially expanded above each 

 vertebral centrum, thus leaving a vaulted chamber in the united neural 

 arches of the sacral vertebra?. A cast of this cavity in the type speci- 

 men of Apatosaurus is shown in fig. 3, PI. XVIII. 



