258 IEC, CASS. 
Pelycosauria.—Among the specimens in the Chicago collec- 
tion is a single sacral vertebra which presents some very inter- 
esting points. It is the anterior one of the sacral series, and to 
one side is attached the sacral ribs in the natural position. The 
rib is united by a close articulation to the centrum, and the distal 
face is much expanded and turned somewhat to the rear. The 
lower edge of the centrum is preserved, and shows that it had 
the same peculiar oblique form characteristic of the cervical and 
sacral series of the Texas Pelycosauria. But the main interest 
centers in the neural arch as shown in Figs. 3 and 4, Plate I. 
The base of the neural arch is perorated on each side by a 
large foramen. Between the posterior zygapophyses a stout 
ridge extends up the middle of the spine, and there is a rudi- 
mentary articular process at the base of the ridge. The upper 
half of the posterior face of the centrum is broad and flattened, 
and evidently served to form a very strong articulation with the 
second sacral. This seems to indicate the beginning of asacrum, 
or it may be pathological. Unfortunately the specimen was iso- 
lated when found, so that the relation to the succeeding verte- 
bre cannot be exactly determined. 
The pelvic and thoracic girdles of the American Pelycosauria 
have as yet been almost unknown. Cope published a figure of 
the scapula and coracoid of Dimetrodon and the author pub- 
lished with Dr. Baur a figure of a second specimen of the same; 
both of these specimens were somewhat incomplete. In 1886 
Cope published a figure of the interclavicle of a Pelycosaurian. 
A peculiarly perfect specimen in the Chicago collection makes it 
possible to restore the thoracic girdle with much certainty. 
There are preserved the clavicles of both sides, the interclavicle, 
the scapula with attached coracoid and epicoracoid bones of one 
side and less perfectly the same bones of the other side. One 
of the clavicles lies upon the outer, under, side of the anterior 
expanded end of the interclavicle and the other lies upon the 
scapula. 
The interclavicle is somewhat broken and the outer edges of 
the expanded portion are broken away in places, but the main 
