MORPHOLOGICAL REVISION OF THE SUBORDER. 127 



Measurements. — No. 112 University of Chicago — Continued. 

 Dimensions of a chevron from, an anterior caudal. 



mrn. myn. 



Antero-posterior diameter of head 7.5 Distance from top of ilium down to edge of 



Transverse diameter of head ■22 ischium 195 



Length, distal end missing 46 Length of ilium, antero-posterior 155 



Total length, pubo-ischiatic symphysis 270 Greatest length of acetabular c vity 65 



Dimetrodon Incislvus Cope (plates 12-16 and 20). 



Clefsydrops limbatus Cope, p. 40. Dimetrodon rectiformis Cope, p. 49. Dimetrodon semi- 

 radicatus Cope, p. 50. 



C/uiraderistic Specimens : No. 41 16: The type. No. 4008: Co type. No. 4040 

 Am. Mus. Nat. Hist., Cope, coll. : The posterior portion of the vertebral column, 

 including the nine anterior caudals. No. i University of Chicago : An imperfect 

 skull, scapula, vertebral column, and humerus, femur, tibia, and fibula. No. looi 

 University of Chicago : Skull, lower jaws, imperfect vertebral column, shoulder 

 girdle, and fore limbs. No. 1003 University of Chicago: Imperfect fore limb with 

 complete tarsus. No. 4093 Am. Mus. : A pelvis. No. 4092 Am. Mus. : A pelvis, 

 sacral and caudal vertebrse, and four limb bones. No. 4089 Am. Mus. : A pelvis. 

 No. 4039 Am. Mus. : A pelvis, sacrals, and caudals, humerus and femur. 



The main characters of the species have been given in the discussion of the 

 genus. Below are noted the points in which it differs from the related species. 



The outer surface of the maxillary near the tooth line is very rugose and marked 

 with oblique grooves, which run downward and forward (plate 10, fig. 3). The lower 

 jaw is much stouter and relatively shorter than in some species ^^. ^., Dimetrodon 

 longira7niis. 



The vertebrse of the cervical region are nearly upright, the neural arch lies 

 directly over the centrum and the axis of the spine is more nearly vertical ; in Dime- 

 trodon dollovianus the neural arch has the appearance of having been pushed forward 

 so that the anterior cervicals are very oblique (fig. 54). 



The intercentra of the anterior cervicals differ from those of other species in that 

 they are free from the succeeding centrum, in Ditnetrodon gigas the suture between 

 the intercentrum and the lower portion of the articular face of the centrum is hardly 

 distinguishable. In the posterior cervical series the intercentra are free in all species. 

 The spines show a characteristic marking best shown in specimen No. 100 1 Univer- 

 sity of Chicago. The spine of the fourteenth in series, a mid-dorsal, is described as 

 typical (plate 16, fig. 2). The base of the spine at the point of leaving the neural arch 

 is very thin and elongate antero-posteriorly. This shape continues upward some 

 distance (40 mm.) above the neural canal, and then there is a contraction to a nearly 

 round or slightly transverse oval section. The contraction is sudden and from both 

 anterior and posterior sides, so that it has the appearance of a knot or joint, which is 

 heightened by a sculpture of deep and sharp rugose grooves and ridges. This change 

 of shape, with its attendant rugosity, appears on all the spines at about the same 

 height. The upper portion of the spines has grooves on the anterior and posterior 

 side, but these are not deep and disappear half-way up. The upper fourth is very 

 irregular in section, which is partly due to pressure, but not entirely ; the most perfect 

 spines have a rounded oval section with low anterior and posterior edges taking the 

 place of the grooves of the lower half Throughout their length the spines are 

 marked by a fine sculpture of vertical lines, which become finer and more definite 

 toward the apex. 



