20 MEMOIRS OF THE CARNEGIE MUSEUM 
ess of the sacral is stronger not only than those of the preceding and succeeding 
vertebrae, but it is stronger than its sacral rib, although the latter is more robust 
than the movable ribs borne by the transverse processes of the immediately preced- 
ing and succeeding vertebree. It would seem more probable, therefore, that the 
smaller and more slender sacral rib would become obsolete than the stronger 
and more robust transverse process. Whether this elimination was accom- 
plished by the complete disappearance of the sacral rib or by its fusion with 
the transverse process cannot be told. It by the latter process, however, the 
so-called sacral ribs in the Sauropod sacrum would then be homologous with 
the coalesced sacral ribs and transverse processes. But in the sacra of the 
Sauropoda and other highly specialized terrestrial vertebrates, whether reptiles 
or mammals, it would appear to be quite evident that in all those sacral 
vertebree added to the primitive sacrum through the modification of anterior cau- 
dals it is the transverse processes (united diapophyses and paropophyses) that have 
been modified to give support to the ilia instead of true sacral ribs homologous with 
the free ribs borne at the extremities of the transverse processes in the anterior cau- 
dals of Menopoma, for in no instance are the transverse processes of the anterior cau- 
dals of even moderately specialized terrestrial vertebrates known to have borne such 
ribs. Even in the modern Iguana and in the crocodiles where the sacrum is still 
exceedingly primitive consisting of only two ununited vertebre ,there are no movy- 
able or other ribs on the transverse processes of the anterior caudals and none are 
known to the present writer even among the earliest known Dinosauria. It does 
not seem at all reasonable to suppose that these ribs were present in the primitive 
forms in the caudal region, that they disappeared and then reappeared in the suc- 
cessive caudals as these were added to the primitive sacrum more especially since 
their presence would tend to produce instability rather than strength in that region 
where rigidity is especially advantageous. In Figs. 5 and 6 are given superior 
views of the sacra together with the immediately preceding and succeeding verte- 
bree in Menopoma allegheniensis and Iguana tuberculata. A study of these figures 
shows the marked difference in the structure of the sacrum in the two. In Meno- 
poma the ilia articulate with the transverse processes of the solitary sacral through 
the intermedium of sacral ribs, while in the Iguana this articulation is directly with 
processes firmly fixed one on either side of the centra of each of the two sacrals. 
Whatever the exact homologies of these latter processes may be it is impossible to 
say with certainty, though embryology ought to offer some evidence. In general 
form and in position, however, it is evident that they approximate much more 
closely the transverse processes than true sacral ribs. If, however, they are homolo- 
