316 MEMOIRS OF THE CARNEGIE MUSEUM. 
and evidently immature, having lost both the anterior and posterior epiphyses 
of the centrum as well as the greater part of the neural spine. It has been referred, 
because of the greater backward inclination of the spine, to M. petersoni. No. 
1730 is a first sacral, likewise referred to M. petersont. It has also lost the epiphyses 
of the centrum, the upper part of the neural arch, and the whole of the neural 
spine. Specimen No. 2102 consists of the first sacral, lacking the greater part of 
the neural spine, and the centrum of the second sacral thoroughly codssified with 
the centrum of the preceding vertebra. It shows some evidence of being water- 
worn. 
The sacrum is composed of four vertebrae. In adult specimens the vertebrae 
are well coéssified. The spines slope strongly backward and rapidly increase in 
transverse diameter at their extremities to the posterior margin of the third verte- 
bra, and then rapidly diminish to a knife-edge at the posterior margin of the 
fourth vertebra. There is considerable diversity in the antero-posterior diameter 
of the neural spine of the first vertebra. The diameter is greater in M. elatus than 
in M. peterson’. In the latter species the backward inclination of the spines is 
greater than in M. elatus. In specimen No. 1706 (the mounted specimen) asso- 
ciated with the bones listed under No. 1604, the vertebre are thoroughly codssified 
at their centra and above at their extremities. Between the posterior margin 
of the spine of the first vertebra and the anterior margin of the spine of the second 
vertebra there is in this specimen a lacuna about the middle, where the spines 
have failed to codssify. In specimen No. 1704 the spine of the first sacral has 
failed to codssify at its base with that of the succeeding vertebra, and there is in 
consequence an opening revealing the upper part of the neural arch of the second 
sacral behind the spine of the first sacral. The centra are greatly compressed 
vertically, and rapidly diminish backward both in vertical and transverse diameter. 
The neural canal likewise rapidly diminishes in the sacrum from before backward 
and is not so nearly circular in outline as in the dorsals and lumbars, but becomes 
more or less subtriangular, the transverse diameter being relatively considerably 
greater than the vertical diameter. The transverse processes or alee which look 
outwardly and backwardly at their ends flare widely in order to form attachments 
with the ilium, with which in thoroughly adult specimens they are more or less 
firmly united. This lateral enlargement is greatest in the case of the first and 
second vertebre, the transverse processes of which furnish the largest surface for 
adhesion to the ilium. There are three well developed ventral and dorsal foramina 
on either side, of which the anterior is much the largest, the ventral, viewed from 
below, being antero-posteriorly elongated and oval. The opening of the second 
