46 REVISION OF THE PELYCOSAURIA. 



Dimetrodon gigas Cope. 



Clepsydrops giffas Cope. Am. Nat., vol. xii, 1878, p. 327. 

 Proc. Am. Phil. Soc, vol. xvii, 1878, p. 515. Also Pal. Bull. 29. 

 Proc. Am. Phil. Soc, vol. xix, 18S0, p. 43. Also Pal. Bull. 32. 



Type : The head of a humerus, the pelvis, both femora, several dorsal, sacral 

 and caudal vertebrae, and two phalanges. No. 4006 Am. Mus. Nat. Hist., Cope, coll.; 

 from Texas. 



Original description : "This animal is only represented in my collection so far 

 by a large part of the pelvis. This is of the same character as that of the C. natalis, but 

 differs in several details of fonn and is three times as large in linear measurements. 

 The portion anterior to the acetabulum is shorter than in the C. natalis, and relatively 

 deeper. The raised borders of the acetabulum unite, and form a thick obtuse hori- 

 zontal crest, which continues to the apex, which consists of a broadly expanded 

 shovel-like projection. This .symphyseal portion is quite elongate, and carries on its 

 supero-anterior face an obtuse median keel. The opposed elements diverge above 

 the anterior part of the acetabulum. The latter is shallow, but entire ; its most 

 prominent borders are the anterior and postero-inferior." 



Measurements. 



m. m. 



Length from posterior border of acetabulum Total vertical diameter to superior border of 



forward 0.200 acetabulum 0-155 



Long diameter of acetabulum (right side) 100 Length of anterior symphysis 175 



In 1880 Cope added to the description oi D. gigas as follows : 

 "The pelvis of the D. gigas is in general like that of Clepsy drops natalis. The 

 elements are coossified, but the ischio-pubic symphysis is not so deep as in the 

 Batrachia of the same beds. The ilium is shortened, and its direction is at right 

 angles to the long axis of the inferior elements. The foramen of the internal femoral 

 artery is distinct. The femur of the same individual of D. gigas has no head, but a 

 regular wide crescentic proximal articular surface. Below this on the posterior side 

 is the large trochanteric fossa, which is bounded by lateral ridges, which are at first 

 equal, but one soon exceeds the other in height, forming a trochanteric ridge a little 

 above the middle of the shaft. The condyles are distinct from each other and are 

 flattened below. One of them bears a robust longitudinal crest above, which makes 

 it much larger than the other, and causes the groove that separates them above to 

 look outward or to the side which supports the trochanter. 

 " Three of the species may be distinguished as follows : 



" Vertebral centramuch compressed, acute below ; neural spines without processes D. incisi'vus. 



" Vertebral centra less compressed, obtuse below ; neural spines without processes ; larger... £>. g'lg'as. 



"Vertebral centra compressed, not acute below; neural spines with cross projections D.cruciger." 



Revised description : 



(i) Spines quadrangular, nearly square at base ; changing to rounded above ; 

 large, surface not striate. 



(2) Third to sixth cei-vicals with axis through neural arch and middle of 



centrum vertical. 



(3) Intercentrum small in mid-dorsal and lumbar regions, without terminal 



facet for capitulum of rib. 



(4) Character not shown. 



(5) Size, largest of genus. 2.5 to 3 meters in length. 



