A DESCRIPTION OF THE GENUS DIMETRODON, COPE. 49 



process ; the pits above and below it are deep and the supporting ridges are slender. The 

 process stands out nearly straight from the base of the spine. 



The seventh dorsal closely resembles the sixth. The spine is practically complete ; 

 it is over twenty-four times the greatest diameter of the centrum with a length of .830 m. 



The eighth dorsal shows a marked change in the transverse process which is directed 

 forwards instead of straight outwards or backwards as in the anterior ones of the series. 

 The distal end reaches in front of the anterior end of the centrum. The articular face 

 for the tuberculum looks forwards and downwards, the face which formerly ran to join the 

 capitulum has become a short process extending from one side of the tubercular face. 

 The excavations of the side of the centrum have involved more than one-half of the 

 vertical height, limiting the rounded part to the walls of the chordal canal. The 

 descending flange of the articular face occupies fully one-third of their height and the 

 edges have become very thin. The profile retains the sigmoid outline. To the anterior 

 face of the centrum is attached the slightly displaced intercentrum. It is crescentic in 

 outline and narrow from before backwards. The upper, concave part is divided into 

 two nearly equal faces for the adjoining vertebrae. The lower surface is rugose and the 

 upper posterior edges bear well-defined facets for the capitula of the ribs. 



The ninth dorsal (PL III, Fig. 40) preserves both transverse processes entire. The 

 connecting face between the capitular and the tubercular faces has entirely disappeared 

 on the left side and is very small on the right. The upper edges of the anterior faces of 

 the transverse processes have expanded forwards to join the prezygapophyses and roof 

 over a deep pit below. The spine is nearly perfect. It ends in a slightly expanded 

 rugosity. It is .863 m. long, or twenty-six times the greatest diameter of the 

 centrum. 



The tenth dorsal (PL II, Figs. 25, 26) has a more compressed body, the base of the 

 transverse process is thin and expanded anteroposteriorly. The process curves forwards 

 and reaches well in front of the anterior edge of the centrum. The posterior end of the 

 centrum extends. below the anterior end and causes the keel to slant toward the rear. 



The eleventh dorsal has a very deep and narrow keel. The descending flanges on 

 the faces of the centra take up nearly one-half of the vertical height. The posterior 

 articular face extends below the anterior, giving the same oblique direction to the keel as 

 in the preceding vertebra. An intercentrum belonging either at the posterior or the 

 anterior end of the vertebra shows two well-developed faces for the ribs. 



This is the last of the well-preserved dorsals. Posterior to the eleventh are five 

 vertebras that are badly injured by decay. In all of these the keel is very sharp and is 

 inclined toward the rear. In the next to the last the anterior face is steeply inclined to 

 the rear as it descends. The last two show round articular faces with only a very short 



a. p. s. — vol. XX. G. 



