178 FIELD COLUMBIAN MUSEUM. « 
SACRAL VERTEBRA. 
The sacrum in this specimen is composed of five vertebre, 
coalesced by the centra, zygapophyses, and more or less by the sacral 
ribs. The second, third, and fourth vertebre are in addition firmly 
conjoined by their neural spines, the crests of their neural arches, and 
the bases of their diapophyses. Their sacral ribs unite distally to 
form a yoke-like synostosis, which, in the Opisthoccelia, may fittingly 
be termed the sacricostal yoke. This is approximately symmetrical to 
the transverse axis of the sacrum and is borne upon the rim of the 
acetabuluin, which constitutes its chief support. These three vertebrez 
have been described * by Osborn in ‘‘Camarasaurus’’ and by Williston 
in Morosaurus f as constituting the primitive sacrum. They will there-. 
fore be designated in this paper as the primary sacrum, but in the 
series of vertebre functional as sacral, or ilium-supporting vertebre 
they will be numbered two, three, and four. ‘The first vertebra which 
functions as a sacral, but varies in the structure of its sacral rib, will 
be termed the dorso-sacral. In like manner the fifth of the series 
will be termed the caudo-sacral. (Plate x.viit.) 
The dorso-sacral bears evidence of having united with sacral 11 by 
its centrum and zygapophyses before adult age. The sacral ribs show 
a degree of coéssification similar to that between the sacral ribs and 
the right illum. The spine remains free, but stands close to the 
second spine—a condition which would tend to coalition. - The caudo- 
sacral shows even more recent co6ssification with the primary sacrum. 
The centra are’ firmly coéssified, the sacral ribs incompletely so; the 
zygapophyses are conjoined, but little atrophied, the neural spine 
remains free and isolated. A comparison in structure between this 
vertebra and the first caudal shows no greater difference than that 
between caudals 1 and 1. | 
The centrum in sacral 1 is not reduced in size and differs from the 
dorsal centra only in its thicker walls. The anterior end is slightly 
convex; the lateral cavities open just behind the base of the sacral 
rib. (Plate xLvut.) The centra of sacrals 1, 11, 1v are considerably 
reduced in size and are more firmly united. The pleuro-central cavities 
open upward, just back of the pedicles. The centrum of sacral v is — 
firmly united with the preceding one, but is not reduced in size and is 
little excavated by lateral cavities. The posterior surface is quite 
concave in its lower half, the margin slightly receding above. 
The neural arches begin with the dorsal type in sacral 1; in I, 111, 
and iv they are drawn together antero-posteriorly and conjoined from 
* Bulletin Am. Mus., Vol. x, p. 230. 
+ Kans. Univ. Quarterly, Vol. vii, p. 173. . 
