— 3145 — 
Thoraco-lumbar region. 
The thoraco-lumbar vertebrae number 18, or four less than in 
Stereosternum tumidum, a most astonishing disparity in two forms 
so closely related. This portion of the spinal column measured—with 
some allowances for displacements—in three specimens, 133 mm., 
150 mm., and 163 mm, All the thoraco-lumbars bear high, longi- 
tudinally expanded neural spines (Pl. IV., fig. 10) and the neural 
arches are greatly swollen and produced latero-posteriorly into heavy 
metapophyses (Pl. Il., fig. 2). The zygopophysial facets are nearly 
horizontal. The actual centra are quite small, but expanded laterally 
into heavy transverse processes (Pl. IIl., figs. 3 and 4), aS in other 
Mesosauridae. In the neck and anterior thoracic regions these pro- 
cesses are slightly further forward on the vertebrae than is the case 
in the lumbar region. 
Ribs. 
The ribs are very similar to those of the other known members 
of thefamily. Thesingle head articulates rather loosely with a shallow 
depression, scarcely a facet, on the under side of the heavy trans- 
verse process. Passing backward the rib articulations recede from 
the anterior to the middle region of the corresponding vertebra. The 
very great thickness of the ribs, especially of the distal half, is 
very characteristic of all known Mesosauridae, and recalls the some- 
what similar condition of the ribs in the mammalian order Sirenia. 
Of the eighteen thoraco-lumbar ribs in the present species all but 
the last three seem to be truncated at their distal ends, and appa- 
rently were tipped by cartilages, and very probably thus connected 
with a cartilaginous sternal apparatus, but nocartilage has been fos- 
silized in any of thespecimens. The last two or three ribs are shorter, 
pointed distally, and there is reason to believe that their vertebral arti- 
culation was somewhat more firm than in those more anteriorly 
placed. 
Sacrum. 
There are tivo permanently separate sacrals, as in Ster‘eosternum. 
In the adult the broad sacral ribs seem to be anchylosed with the 
vertebrae, but there is some evidence that they are rather readily se- 
