MORPHOLOGICAL REVISION OF THE SUBORDER. 



117 



lies at a higher level than that of the maxillary. The change from one level to another 

 is accomplished abruptly, just anterior to the enlarged maxillary tooth. All the bones 

 of the skull are heavier, more clumsy, and are marked by rugosities and pittings more 

 strongly than in Dimetrodon incisivus. 



The vertebral column : The inferior surface of the preatlantal intercentrum is 

 rough and the facets for the capitula of the ribs are not so well marked as in Dime- 

 trodon incisivus. 



The atlas is similar to that of Dimetrodon incisivus, but is proportionately broader 

 and shorter. The anterior face is nearly as broad as high. The opening for the 

 notochord on the posterior face is notably smaller than in Dimetrodon incisivus and 

 Dimetrodon obtusidens. 



Fig. 42.— Axi» of D. gigas. No. 4028 Am. Mus. X %. 

 Fig. 43.— Axis of D. gigas. No. 4147 Am. Mus. X }4. 

 Fig. 44.— Spine of axis of D. gigas (?). Nos. 4054, 4055 Am. Mus. 



X 'A. 



Another atlas, No. 4034 Am. Mus., has the anterior face 38 mm. high and the 

 transverse diameter the same. 



The neural arch is lost in this specimen, but in a very incomplete specimen of 

 this species. No. 4034 Am. Mus., the neural arch is preserved. It has the same form 

 as in Ditnetrodon incisivus, but in common with the rest of the skeleton is stronger 

 and more rugose. The groove above the flat facet on the outer edge is almost con- 

 verted into a foramen (plate 21, fig. 8). 



The second intercentrum is smaller than the first. 



The axis has a broad and high spine, the apex is widened and thickened for 

 ligamentous attachment, and the anterior edge is rugose for the same purpose, but there 

 is no distinct face on the anterior edge as in Dimetrodon dollovianus (plate 21, fig. i). 

 There are several specimens of the axis in the two collections. 



No. 1002 University of Chicago, fig. 34. Height of spine above center of neural 

 canal, 87 mm. 



