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PALEONTOLOGY: S. W. WILLISTON 
SPHENACODON MARSH, A PERMOCARBONIFEROUS 
THEROMORPH REPTILE FROM NEW MEXICO 
By Samuel W. Williston 
WALKER MUSEUM. UNIVERSITY OF CHICAGO 
Received by the Academy, October 23,1916 
Thirty-eight years ago, 0. C. Marsh gave the name Sphenacodon 
ferox to some jaws and vertebrae collected by David A. Baldwin near 
Arroya de Agua in Rio Arriba County, New Mexico. From his brief 
description it was suspected that the form was nearly allied to the re- 
markable reptiles known as Dimetrodon Cope, from the Permian of 
Texas, and a study of the type collections in the Yale Museum enabled 
me a few years ago to confirm the suspicion. The locality whence the 
type specimens came had almost been forgotten till five years ago, when 
Mr. Paul Miller, Prof. E. C. Case and I rediscovered the original bone- 
bed of Baldwin about a mile and a half above the confluence of the 
Poleo creek with the Puerco, and near the Mexican village of Arroya 
de Agua. Numerous specimens were obtained from this deposit, fixing 
the identity of the forms with those of other remains obtained from a 
bone-bed in the same horizon and a half mile nearer the mouth of the 
Poleo. From this bone-bed or 'bone-quarry,' discovered by Mr. Miller, 
numerous isolated bones of Sphenacodon and Ophiacodon Marsh, to- 
gether with a very perfect skeleton of the latter, were obtained, but 
lack of time prevented a thorough exploitation of the deposit. 
The past season my son, Samuel H. Williston, and I spent a few 
weeks in the same region, securing some very interesting and important 
specimens, a detailed description of which will be published later, includ- 
ing the material upon which the accompanying figure is based. 
The ^Miller bone-bed' is in a hard, dark brown clay, which is worked 
with some difficulty because of the heavy embankment over it. It 
extends for about 100 feet along the foot of the high hills bordering the 
Poleo, and bids fair to yield not a few more good skeletons. About 20 
feet east of the spot that yielded the remarkable skeleton of Ophiacodon 
that has been described, a femur of Ophiacodon was observed protruding 
from the excavation made by us in 1911. Upon following it up, two 
skeletons were discovered, one of Ophiacodon, lying immediately upon 
another of Sphenacodon, with their ends reversed. The extreme tip of 
the skull of Sphenacodon had been removed and lost in our previous 
excavations. It is altogether possible that I am the one who is respon- 
sible for the mutilation, but I am quite willing to share the responsi- 
bility with Mr. Miller and Prof. Case! 
