536 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 56. 



The ulna has a greatest length of 236 mm.; greatest diameter of 

 proximal end, 68 mm.; greatest diameter distal end, 41 mm. 



The radius has a length of 190 mm.; greatest diameter proximal 

 end, 50 mm.; greatest diameter distal end, 48 mm. 



According to Case the femm* of Dimetrodon gigas has a heavier 

 outer condyle and a slightly curved shaft, which distinguishes it 

 from D. incisivus. I fail to note any difference except that of size 

 in the femur of No. 8635, U.S.N.M., when compared with femora 

 of D. incisivus. The femur has a greatest length of 240 mm.; 

 greatest width of distal end, 90 mm.; of proximal end, 79 mm.; the 

 least width of shaft, 26 mm. 



Like the femur the tibia shows no characters peculiar to this 

 species. The greatest length of the tibia is 195 mm. 



Carpus and forefoot. — The left forefoot of specimen No, 8635, 

 U.S.N.M., appears to be the most complete manus of a Dimetrodon 

 that has yet been discovered, and the first to be described of the 

 species D. gigas. Fortunately the greater number of the bones 

 comprising it were found articulated, thus giving positive evidence 

 of their proper association and arrangement as shown in figure 8. 



Case,* it appears, was the first to figure and describe the articu- 

 lated carpus and portions of attached metacarpus of this genus, 

 based on a D. incisivus foot. 



In 1911 Willis ton ^ published a figure of an articulated carpus of 

 this same species giving an anterior view. 



These references cover practically aU that has been written on the 

 feet of Dimetrodon. The hind feet are as yet practically unknown. 



The carpus in the present specimen is complete except for the ele- 

 ment designated by Case the "sesamoid," which articulates with 

 weU-defined facts on the radiale side of centrale 2 and carpale 1. It 

 has been restored here (see s, fig. 8) after an element belonging with 

 a forefoot of D. incisivus kindly loaned me by Dr. S. W. Williston. 

 Metacarpals 3, 4, and 5 and proximal phalangials 1 and 2 of digits 

 4 and 5 were found articulated or so little displaced as to raise no 

 doubt as to their being in proper sequence. Other bones that were 

 detached but found associated in the same field packages were meta- 

 carpal 1, proximal phalangials of digits 1, 2, and 3, and two ungual 

 phalanges, here arbitrarily placed on digits 4 and 5. (See fig. 8.) 

 All of these detached elements probably pertain to this foot, but of 

 phalangial 2 of digit 2 and coossified phalangials 2 and 3 of digit 3, 

 and the unguals of the first tliree digits, one can not be so sure they 

 pertain to this same individual. However, the final test of the cor- 

 rectness or incorrectness of the association of these elements as 



1 Case, E C. Joum. of Geology, vol. 11, No. 1, p. 11, 1903, and Pelycosauria of North America, Camegie 

 Institution, Pub. No. 55, 1907, pp. 113-114, pi. 16, fig. 1. 



2 Williston, S W. American Permian Vertebrates, 1911, pi. 7, fig. 8. 



