o6 AMERICAN PERMIAN VERTEBRATES 



the arch, with both of which they are closely fused, with only indi- 

 cations of the distinguishing sutural attachments. A few milli- 

 meters from the origin the rib contracts into a stout trihedral 

 or prismatic shape, and then immediately expands into a wide 

 extremity with a periphery of about three-fifths of a circle whose 

 chord is above, hollowed into a deep cavity which looks nearly 

 upward in the articulated skeleton. The rib is so directed that the 

 plane of the anterior end of the centrum passes through the middle 

 of the cavity and the posterior fifth of the acetabulum. The second 

 pair of ribs are more slender and are directed more obliquely for- 

 ward. The base, attached as in the first pair, is less robust, the 

 shaft more slender, and the simple terminal expansion curves some- 

 what downward to be attached to the posterior expansion of the 

 ilium, wholly back of the acetabulum; the extremity is suturally 

 attached at the anterior corner with the posterior part of the end 

 of the first rib. 



Caudal vertebrae (Plate III). — Forty-seven caudal vertebrae 

 were found associated in the mounted specimen in an uninterrupted 

 and nearly straight series, and they have been mounted practically 

 without disturbance of the matrical attachments. Perhaps a 

 half-dozen of the minute terminal ones were missing. The centra 

 throughout are nearly uniform in length. Chevrons were found 

 associated with many, if not the most, of the vertebrae, but the 

 delicate structure of many of the smaller ones rendered it inexpedi- 

 ent to attempt their recovery; some of those posteriorly were 

 appressed against the centra and have been left in that position. 

 Figures are given in the plate of those chevrons recovered in this 

 specimen, and in the photograph of the skeleton others are seen 

 that have been modeled to correspond with isolated or attached 

 ones. 



The first four or five centra have a rather sharp keel below, 

 becoming a little more obtuse posteriorly. By the sixteenth or 

 seventeenth the centra have lost nearly all of the lateral concavity 

 or depression, and thenceforth a cross-section through the middle 

 forms a rather regular semioval figure. Between the second sacral 

 and the first caudal there is space for a small intercentrum. Below 

 the first and second caudals the space is moderately large, yet 



