O. C. Marsh—American Jurassic Dinosaurs. 87 
ankylosed to the centra. Those on each side are united distally 
into a solid mass, which rests on the short ilium. The articular 
faces of the sacral vertebre are nearly plane. That of the ante- 
rior centrum is a transverse oval in outline, and the posterior 
face is more nearly round. The centra and their processes are 
somewhat lightened by cavities, as in the sacra of Adlantosaurus 
and Morosaurus. The sacrum of the latter genus, shown in fig- 
ure 2 of Plate V, is built upon the same general plan, character- 
ess 
are found in the same localities. The sacra show the genera to 
be quite distinct, and the abundant material now in the Yale 
_ Museum, when carefully collated, will enable other parts of the 
_ structure to be compared. The teeth in all the herbivorous 
genera of the Sauropoda from the Atlantosaurus beds, so far as 
now known, appear to be very similar, and hence do not afford 
generic characters. — ; 
e type species of the present genus is Apatosaurus ajax 
Marsh, and the known remains indicate a reptile at least fifty 
feet in length. A much larger species is indicated by various 
remains from the same locality in Colorado, among which 1s 
the huge cervical vertebra represented in Plate III, figures 1 and 
2. This species had a short massive neck, and hence may be 
* This Journal, vol. xvi, Plate VI. 
