190 DINOSAURS OF NORTH AMERICA. 



a similar large chamber in the same position. The form and propor- 

 tions of this cavity are indicated in PI. XL VI, figs. 2 and 3, which 

 represent a cast of the entire neural canal inclosed in the sacrum. 

 The large vaulted chamber, it will be observed, is contained mainly in 

 the first and second sacral vertebra;, although the canal is consider- 

 ably enlarged behind this cavity. The sections represented in fig. 4 

 are in each case made where the transverse diameters are greatest. 



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, which is also indicated in the same plate, and in this respect 

 it is entirely without a parallel. A perceptible swelling in the spinal 

 cord of various recent animals has indeed been observed in the pectoral 

 and pelvic regions, where the nerves are given off for the anterior and 

 posterior limbs; and in extinct forms some very noticeable cases are 

 recorded, especially in dinosaurs, but nothing that approaches the sacral 

 enlargement in Stegosaurus has hitherto been known. The explanation 

 may doubtless in part be found in the great development of the pos- 

 terior limbs in this genus; but in some allied forms — Camptosaurus, for 

 example, where the disproportion between the fore and hind limbs is 

 quite as marked — the sacral enlargement of the spinal cord is not one- 

 fourth as great as in Stegosaurus. 



It is an interesting fact that in young individuals of Stegosaurus 

 the sacral cavity is proportionately larger than in adults, which corre- 

 sponds to a well-known law of brain growth. 



The physiological effect 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, that in an animal so endowed 

 the posterior part was dominant. 



THE PELVIS. 



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 shallow (PI. XL VI, fig. 5). The face for union with the ischium 

 is large and rugose, but that for the pubis is much less distinct. The 

 postacetabular part of the ilium is very short, scarcely one-third as 

 long as the anterior projection. 



The ischium of Stegosaurus ungulatus is short and robust, and has a 

 prominent elevation on the upper margin of the shaft (PL XLVI1I, 

 fig. 2). Its larger articular face meets a postacetabular process of the 

 ilium, and a smaller articulation joins the pubis. The shaft of the 

 ischium is twisted so that it resembles somewhat the corresponding 

 bone of Morosaurus. The pelvis of Stegosaurus stenops is shown in the 

 same plate, tig. 3. 



