STRUCTURE AND RELATIONSHIPS OF OPISTHOC(ELIAN DINOSAURS. I81 
family, all of which present very distinct points of similarity. It is 
probable, as evidenced by the separated sacral vertebre in M/orosaurus 
lentis that the condition of ossification varies with age, the middle three 
uniting earliest, the first next, and the fifth last. The slight union of 
the fifth might, indeed, be absent in the adult without affording 
generic or even specific characters.’’ * 
A year later Dr. Osborn wrote of the sacrum of D7plodocus: *‘ There 
are four rib-bearing true sacral vertebree in Dzplodocus instead of three, 
as hitherto described by Marsh. The three anterior sacrals, consti- 
tuting the primitive Dinosaurian sacrum, are firmly united by their 
neural spines. These three spines coalesce into a single very robust 
spine showing the diapophysial lamine separate.’’ 
In his recent monograph on Dip/odocus Mr. Hatcher described 
two sacra as follows: **The two splendid sacra belonging with skele- 
tons 84 and 94 in our collections are unusually complete. .... In 
each instance the vertebre are firmly codssified with and give support 
to the ilia. In 84 the right ilium alone is preserved and this is united 
with all five of the vertebrz which function as sacral either by means 
of true sacral ribs or the expanded diapophysial amine or by both 
these elements. All are codssified. by their centra and the three median 
have their spines coalesced. . . . . Inskeleton 94, however, there are 
noticeable certain other more marked differences which are worthy of 
especial notice as bearing directly upon the nature of the primitive 
Dinosaurian sacrum. In this skeleton the sacrum is present, with 
both ilia in position. The centra of the true sacrals are all codssified 
asin othersacrum. The neural spines of sacrals one and two coalesce 
and are coossified throughout their entire length as in 84, but the spine 
of sacral three is quite free from, though closely applied inferiorly to, 
that of the second sacral. This would seem to indicate that the prim- 
itive Dinosaurian sacrum consisted of two rather than three vertebre, 
a condition found in the Crocodilia and most other living Reptilia. 
The fourth sacral in No. 94 bears a free spine and is codssified by its 
centrum with the third and does not differ in any essential respect from 
‘that described by Osborn or from that found to obtain in No. 84 of 
our collections.”’ 
From the above it will be seen that Osborn and Williston agree 
upon the structure of the primitive sacrum as indicated by ‘‘Camara- 
saurus’’ (Apatosaurus), JZorosaurus, and Diplodocus. This is based 
upon the tendency of the median three vertebre to unite by spines and 
ribs. Hatcher has dissented from this opinion, postulating that the 
primitive Dinosaurian sacrum consisted of no more than two vertebre. 
* Kans. Univ. Quarterly, Vol. vii, p. 173. 
