168 O. C. Marsh— American Jurassic Dinosaurs. 
The remarkable feature about this posterior brain-case, if so 
it may be called, is its size, in comparison with that of the true 
brain of the animal, and in this respect it is entirely without a 
not one-fourth as great as in Stegosaurus, 
It is an interesting fact then in young individuals of Stego- 
saurus the sacral cavity is proportionately larger than in adults, 
which corresponds to a well known law of brain growth. 
e physiological effects of a posterior nervous center, so 
many times larger than the brain itself is a suggestive subject, 
which need not here be discussed. It is evident, however, ihe 
in an animal so endowed, the posterior part was dominant. 
Tue Peretvic Arca. 
The true sacrum of Stegosaurus is composed of four well 
coéssitied vertebra. In fully adult animals the pelvic arch 
may be strengthened by the addition of one or more lumbar 
vertebra, as in the specimen figured in Plate VII, where two 
are firmly consolidated with the sacrum. The centra of the 
sacral vertebre are solid, like the others in the column. Their 
neural arches are especially massive, and the spines have high 
and expanded summits. e transverse processes of the sacral 
vertebrae are stout vertical plates, which curve downward 
below, and unite to meet the ilia. Each vertebra supports 1ts 
own process, although there is a tendency to overlap in_ front. 
There is a gradual increase in size from the first to the last 
sacral vertebra, and the first caudal is larger than the last 
sacra 
The ilium in Stegosaurus is a very peculiar bone, unlike any 
hitherto known in the Reptiles. Its most prominent feature 
is its great anterior extension in front of the acetabulum. 
Another striking character is seen in its superior crest, which 
curves inward, and firmly unites with the neural arches of the 
sacrum, thus roofing over the cavities between the transverse 
processes. The acetabular portion of the ilium is large and 
* The main facts here presented were communicated to the National Academy 
of Sciences, in a paper read at the New York meeting in November last. 
