184 FIELD COLUMBIAN MUSEUM. 
is codssified by all of the ribs and diapophyses, the left evidently 
remained free. The type specimen of “* Brontosaurus’’ is similar, so 
far as can be determined, except that the first presacral is also con- 
joined (Fig. 9). 
This series shows just such a development as one might expect in 
passing from a two-thirds grown animal to one of advanced age. The 
additional dorsal vertebra united with the sacrum in the type specimen 
of ‘‘ Brontosaurus’’ and the additional caudal united in the Carnegie 
Museum specimen, are but further evidence of the persistent tendency 
of adjacent vertebre to codssify with the sacrum in animals of 
advanced age. 
The four known sacra of Diplodocus show a very similar develop- 
mental series. The sacrum of D. /ongus, figured by Marsh, has only 
three vertebre’ coalesced by centra and by sacral ribs (Fig. 10). The 
posterior end of the sacricostal yoke shows a facet for the caudo-sacral 
rib. The Carnegie Museum specimen No. 94 has vertebre Il, I11, Iv, 
and v coalesced by the centra, 1, 111, and Iv by the ribs, and m and 
11 by the spines. The dorso-sacral remains free (Fig. 11). The Ameri- 
can Museum specimen has an arrangement similar to that of the adult 
Apatosaurus, except that the sacral rib in the dorso-sacral is less 
strongly developed (Fig. 12). The Carnegie Museum specimen No. 84 
is described as having this rib more nearly functional (Fig. 13). 
The Morosaur sacrum is known to the writer from four specimens. 
The type of JZ. dentus has four disarticulated sacral centra, each of 
which bear functional ribs not united with the centrum (Fig. 14). The 
neural arches are likewise free from the pedicles. The neural arch 
and spine figured with the posterior centrum in this specimen, and 
which has long been called in question, belongs with a caudal vertebra. 
This is proved beyond question by an almost identical specimen in this 
Museuin. 
The Morosaur sacrum No. 5384 of the Museum has centra 11 and 
i, and tv and v, united in pairs (Fig. 14). The ribs are free from the 
centra as are the neural arches. The spines of 11, 11, and Iv are 
imperfectly coéssified by their zygapophyses. The specimen indi- 
cates an animal little more than half grown. A third Morosauroid 
sacrum in the University of Kansas (figured by Williston /oc. cit.) has 
vertebre I, II, 111, and 1v united by centra, spines 1, tI, and Iv coal- 
esced and the sacral ribs of the same united to form the sacricostal 
yoke (Fig. 15). The broad rib of the dorso-sacral is firmly united with 
this yoke and there is a blunt posterior projection from the fourth rib 
indicating that it abutted against the rib of the caudo-sacral which in 
this form has not been found conjoined. This specimen agrees very 
