186 FIELD COLUMBIAN MUSEUM. 
nating additions on each side of the axis just as the perissodactyl foot 
in mammals has been reduced. With these three vertebre united to 
form the sacricostal yoke the ilium attained a firmness adapted to 
terrestrial habits, and yet sufficiently mobile for bipedal locomotion. 
If we may assume that the Opisthoccelia have passed through such a 
stage, this would account for the sacrum having become stable at this 
point in its development. If the form of the ilium had by that time 
become fixed, the sacrum would have been structurally complete. But 
with adaptation to quadrupedal habits, and the attendant increase in 
size, the need of a mobile sacrum was replaced by the demand for 
greater rigidity. In this way a fourth and fifth sacral, with occasional 
supernumeraries have been added. | 
MODIFICATION IN THE LAST PRESACRAL VERTEBR-. 
The probability that the Opisthoccelian sacrum has expanded by 
the addition of presacral vertebre in front, as well as the addition of 
caudals in the rear, has been doubted by some later writers. The fol- 
lowing features in the specimen under consideration are of interest as 
bearing directly upon this question: 
1. The last pair of presacral ribs tend to codssify with the ver- 
tebra by both head and tubercle. 
2. The distal end of the right rib abutted the inner angle of the 
iliac crest and was evidently attached to it. 
3. The position of the capitular attachinent of the ribs passes by 
. regular gradation from the lateral surface of the centrum in sacral Il, 
to its extreme elevation on presacral I. 
4. The capitular and tubercular elements, as well as the line of 
union with the diapophysis, can be traced in the rib of the dorso-sacral. 
The ast presacral rib on the right side of the Museum specimen is 
firmly codssified by both head and tubercle. The left one is less 
completely so, although the process has gone far enough to have quite 
obliterated the articular facets. The shaft of the right rib, unlike 
that figured by Hatcher in Diplodocus, is barely long enough to reach 
the anterior margin of the illum. The distal end bears an oblique 
facet, which evidently abutted the inner angle of the iliac crest. There 
is no evidence of cod6ssification and no corresponding facet on the 
illum. As imbedded in the matrix, the end of the rib had slipped a 
few inches past the ilium, and the side of the shaft still bears an 
indentation caused by contact with the angle of its crest. An even 
more pronounced instance of modification in a presacral rib was noted 
by Dr. Williston in a specimen of Morosaurus in the Kansas Univer- 
sity Museum. In this case the: distal end: of the rib had become 
