182 : FIELD COLUMBIAN MUSEUM. : 
This conclusion is based upona single sacrum of Diplodocus; a second: 
sacrum: referred to the same species and described in the same paper 
agrees with the usual type in having the spines of sacrals 1, 11, and Iv 
coalesced. The sacrum just described in this paper also agrees in the 
same particular. 
It will be observed that there is a considerable variation in the 
cooéssification of various elements in the Opisthoceelian sacrum, which 
cannot be attributed to age. By comparing the composition of known 
sacra of Apatosaurus, Diplodocus, and Morosaurus, it will be seen 
that there is a noticeable similarity in the development of that of the 
former two, while the latter stands quite aloof. This is well repre- 
sented by the accompanying diagrammatic figures. 
The Afatosaurotd sacrum is represented by four, possibly five, 
known specimens. The most primitive composition is found in the 
young specimen in the Yale University collection, well known as the 
type of Apatosaurus. In this sacrum only the three primary centra 
with their coalesced ribs are preserved (Fig. 5). A second specimen, 
described by Dr. Osborn as Camasaurus,* has, according to his descrip- 
tion, the three primary centra coalesced by spines and sacral ribs as 
well as by the centra (Fig. 6). The dorso- and caudo-sacrals are both 
free; the latter, though structurally a caudal, was functional as an 
ilium-supporting vertebra. These characteristics as well as the size of 
the animal indicate that it was not-quite adult. A third specimen in the 
Carnegie Museum figured + by Mr. Hatcher as *‘ Brontosaurus,’’ may or 
may not belong to this genus (Fig. 8). The figure shows that the 
primary sacrals have their spines coalesced as in the typical adult. 
The caudo-sacral is firmly conjoined by centrum, ribs, and diapophyses 
as is the centrum of the dorso-sacral. The first caudal is also con- 
joined, at least in part. 
The unusually well preserved sacrum in the Museum specimen 
may be taken as a typical representative of the adult of this genus. 
In this, sacrals 11, m1, and 1v are firmly conjoined by centra, neural 
arches, sacral ribs, and bases of the diapophyses (Fig. 7). The centra, 
sacral ribs, diapophyses, and lateral spinous plates are more or less 
reduced from loss of function. The zygapophyses and zygosphenes 
are no longer traceable. The dorso-sacral is firmly coalesced by 
centrum and zygapophyses; the ribs are partially conjoined, the spine 
is free, but stands close to that of sacral 11. The caudo-sacral is a 
typical anterior caudal codéssified by centrum and zygapophyses, and 
partially by the ribs. The spine is free and isolated. The right ilium 
* T0c. cit. 
t Memoirs of the Carnegie Museum, PI. x, Fig. 3. 
