96 REVISION OF THE PELYCOSAURIA. 



The species Dimetrodon tncisivus is tlie most common in occurrence 

 and the best-known member of the Pelycosanria. The description and res- 

 toration of the genns is based largely on this species; a few points as noted 

 below have been supplied from other species. 



Details of the skull have been supplied from Dimetrodon gigas^ No. 

 1002 University of Chicago. The shoulder-girdle is described almost 

 entirely from Dimetrodon dollovtaniis, No. 114 University of Chicago. The 

 posterior foot has been restored from that oi Clepsydrops na talis, No. 41 10 

 Am. Mus. This is the only portion taken from another genus, bvit as the 

 fore foot of Clepsydrops^ No. 2290 Am. Mus., closelj' resembles that oi Dime- 

 trodon it is safe to assume a resemblance in the hind foot. 



The shdl : There are three skulls in the University of Chicago collection belong- 

 ing to the genus Dimetrodon. These supplement each other perfectly, one supplying 

 details lacking in the other. 



No. I University of Chicago, D. incisivus, is an imperfect skull found in a hard 

 joint-clay with the bones disarticulated and scattered, but free from any distortion. 

 It gives most clearly the characters of the basicranial region and the pterygoids, and 

 afforded an opportunity to make a cast of the brain cavity. 



No. looi University of Chicago, D. i>ia'siz'i/s, was preserved in a soft shale which 

 was easily washed from the bones with a soft sponge, permitting the finest details to 

 be made out. The bones were preserved in place, but badly broken and in places rotted 

 by gypsum. It shows the temporal and quadrate regions and the relations of the 

 transverse, the structure of the posterior portion of the top of the skull and the 

 lower jaws. 



No. 1002 University of Chicago, D. gigas, was preserved in a compact red clay, 

 and the bones were covered with a hard scale of calcareous material, which was removed 

 with comparative ease, leaving the bones hard and perfect. This skull is unique in the 

 perfection of its preservation, the only portion missing being the temporal arches, in 

 part, of the left side and the median portion of the epipterj'goids. The skull lay on its 

 side, and all the bones are joined in their natural relations. The whole skull has been 

 crushed slightly from the sides, so that it appears more narrow than it really is. The 

 bones of the top of the skull have been slightly broken and the palate has been pushed 

 slightly downward, but on the whole the skull has been so little changed from its 

 natural condition in life that it is easily restored. 



This specimen has made plain the general proportions of the skull and the bones 

 of the vomerine and ethmoidal region. 



The quadrate, plate 11, figs. 1-4, is a thin plate of bone of considerable vertical 

 extent reaching nearly half the height of the posterior portion of the skull, but not 

 reaching such a great antero-posterior length as the same bone in Sphenodon. The 

 articular portion consists of two condyles elongate in the antero-posterior direction and 

 with their main axes converging slightly as they advance, so that all motion of the 

 jaws was rigidly limited to the vertical plane. The outer condyle is the more slender 

 and lies almost in the plane of the upper portion of the bone ; posteriorly it extends 

 beyond the main part of the bone as a prominent process, with its upper face flattened 

 into a sort of shelf to which is attached the lower end of the quadrato-jugal. The 

 inner condyle is stouter and is offset from the body of the bone. The posterior edge 



