I30 



REVISION OF THE PELYCOSAURIA. 



The vertebrae of the dorsal and lumbar region are very similar to those of Dime- 

 trodoii incisivus. Centrosphenes and centantra are present on all presacral vertebrae. 

 The neural arch is exceptionally high in the dorsals, so that the zygapophyses are 

 farther above the centra than in other species and are very oblique. 



The seven posterior lurabars, the sacrals, and the first caudal are connected. 

 They show the sudden shortening of the centrum and increasing concavity of the 

 mid-line below in the lumbars and the sudden increase in length of the .sacrals better 

 than any other specimen. As the vertebrae become shorter the sides of the centrum 

 are contracted by a deep pit just below the notochordal canal. The pit does not 

 exteud to the lower edge, which remains rounded and without a trace of a keel. The 

 sides and bottom line become very concave, so the edges of the articular faces are pro- 

 portionately much wider flared than on the more anterior vertebrae. The capitulum 

 and tuberculum of the ribs unite and gradually grow smaller, until on the last lumbar 

 they are simple processes. The three sacrals are abruptly longer than the last lum- 

 bar, and the centrum is smoothly rounded, without the deep pits on the sides. The 

 centra fit closely together, and the intercentra are reduced to thin scales anchylosed to 

 the adjacent edges of the centra. The zygapophyses are atrophied and the spines are 

 small and weak. 



The first sacral rib has a wide distal end, the posterior two are smaller. The 

 third is inclined sharply forward and rests against the second, rather as if it acted as a 

 support for the first than as supporting the ilium (plate 23, fig. i). 

 The caudals are as in Dimetrodon incisivus. 



The shoulder girdle: The elements of the shoulder girdle 

 of this species have been used in the restoration of Dimetro- 

 don incisivus. There are few points of difference between 

 the scapula and that of other species determinable from the 

 material at hand. The inner edge of the coracoid and procor- 

 acoid oi Dimetrodon incisivus is nearly straight and joins the 

 anterior edge by a sharp angle ; the same edge in Dimetrodon 

 doUcroianus seems to be more rounded and to join the anterior 

 edge in a gentle curve (plate 23, fig. 2). 



The fore limb is represented by the humerus., the ulna., 

 and an imperfect foot. The ulna is more clumsy, with a 

 heavy proximal end, a more widely open articular cavity, and 

 a less perfect olecranon process ; it is of the type of fig. 56. 



The pelvic girdle: The pelvis is of the same form as 

 Dinietrodo7i incisivus. The ischium and pubis are separated 

 in the specimen (No. 114 University of Chicago) as if they 

 had not been so closely united as in Dimetrodon incisivus. 

 The hind limb is represented by the distal end of the femur and the proximal end 

 of the tibia only. 



No. 4035 Am. Mus. This specimen evidently belongs to the same species as 

 No. 114 University of Chicago. It shows the form of the axis and third cer\'ical 

 (plate 24, fig. i). The sixth vertebra is the first one to take on definitely the character 

 of the dorsals, the thin keel and shortened bottom line. 



The spines of the anterior vertebrae are short, becoming very slender just above 

 the base; the spine of the seventh is abruptly higher than the others, and that of the 

 eighth even more so. The spines of all are imperfect, but are so slender at the ends 

 that it is evident that they are nearly complete. Only the lower portions are repre- 



57 56 



Fig. 56. — Proximal end of an ulna 

 of D. dollovianus. X yi. 



Fig. 57. — Proximal end of an ulna 

 of D. incisivus. X /j. 



