SYSTEMATIC REVISION OF THE SUBORDER. 45 



caudal vertebrte the two facets of the ribs are approximated and finally are not 

 distinguished. They are here coossified with the centra. 



" Vertebrcc. * * * The dentition of these animals was of the most formid- 

 able character, consisting of compressed, finely serrate teeth in the maxillary and 

 dentary bones mingled with huge conic tusks on the middle of the maxillary and 

 anterior end of the dentary, and occupying the whole alveolar edge of the premax- 

 illary. The huge neural spines formed an elevated fin on the back. In a medium- 

 sized specimen of Di met radon incisivus, where the vertebral body is 35 mm. in length, 

 the elevation is 900 mm., or 20 >< times as great. The apex of the spine in this 

 species is slender, and apparently was flexible." 



In 1888 (70) appeared descriptions of the vertebrae and shoulder girdle. 



'■'Ribs. In this genus and in Naosaiirtis the sacral ribs are present as in Batrachia. 

 They are short, and vertically compressed, forming a wedge-like body. 



''Sternum. * * * The anterior two-fifths of the bone is nearly square, and 

 slightly concave above, with three angles, one at each side and one anterior ; the rest 

 contracts posteriorly into a long, narrow, flattened shaft, which constitutes three- 

 fifths of the length. This portion is depressed so that the transverse section is lentic- 

 ular. The lateral edges are acute, and without articular facets of any kind. The 

 distal extremity is first grooved, and then fissured, each half terminating in an obtusely 

 narrow apex which is applied to the other half The surface of this part of the 

 element is longitudinally grooved both above and below. 



"■Clavicles. The clavicles in the genus Dimetrodo7i are well-developed elements. 

 They consist of a vertical narrow and a horizontal expanded portion. The anterior 

 border of the bone is rounded ; the internal border is serrate or semidigitate. If the 

 latter unites with the episternum by suture it nnist be by a very open one. This 

 portion is more expanded than in the monotreme Mammalia ; while the episternum 

 is more produced posteriorly." 



This genus is represented by the largest number of species and by the 

 largest number of individuals in both collections. It seems to have been 

 the most abundant animal of the Texas region. 



Revised description : 



(i) Large diastemal notch with few degenerate teeth or none. 



(2) Maxillary canine and incisor teeth greatly enlarged. 



(3) Teeth with crenate cutting edges. 



(4) Neural arch very early coossified with centrum. 



(5) Very marked change in length of bottom line of vertebrae in different 



parts of column. Posterior lumbars shortened, but without wide faces 

 on lower edge of articular faces of centra. 



(6) Anterior dorsals with sharp narrow median keel and wide intercentral face 



on lower edge of anterior face of centnim. 



(7) Spines sharply recurved in posterior lumbar and sacral region. 



(8) Spine of axis high and broad, reaching forward over atlas. 



(9) Limb bones with well-marked articular surfaces. 



(10) Humerus with well-developed entepicondyle and ectepicondylar process. 



(11) Abdominal ribs absent (not observed). 



(12) Tail relatively short. 



(13) Size varying in different species, from about 1.2 to about 3 meters. 



