A DESCRIPTION OF THE GENUS DIMETRODON, COPE. 53 



and somewhat crescentic in outline. The distal end is less expanded and the single 

 articular face is a shallow pit, oblong in outline. 



Measurements. 



M. 



Total length 146 



Greatest breadth proximal end 042 



Greatest breadth distal end 038 



The tibia (PL III, Figs. 38, 39) is greatly enlarged proximally. The shaft is slen- 

 der and curved and the lower end is only moderately expanded. The cnemial crest is a 

 strong ridge separated from the body of the bone by a deep fossa opening on the outer 

 side. The fossa is continued onto the upper side of the bone as a deep pit which divides 

 the articular face into two unequal halves, connected at their inner ends. These halves 

 are again divided by a low trochlear ridge running fore and aft. On the posterior sur- 

 face of the bone below the outer or fibular edge of the proximal end there is a strong, 

 rounded swelling. The distal end is semicircular in outline, flattened before and rounded 

 behind. There is no indication of a division of the distal end into articular facets. 



Measurements. 



M. 



Total length 177 



Breadth upper end from side to side 072 



Breadth upper end from before back 051 



. Greatest diameter of shaft at centre .021 



Greatest breadth of distal end 044 



Conclusions. 



The description here given of the genus Dimetrodon, together with the described 

 characters of the forms mentioned in the historical review, enable a fairly complete char- 

 acterization of the Pelycosauria to be given. 



Teeth differentiated into incisors, canines and molars. Generally a diastema between 

 the posterior incisor and the canines. The teeth without lateral cusps, but with the edges 

 frequently serrated. The anterior incisors and the canines of the upper jaw much larger 

 than the other teeth. The diastema in the upper jaw marked by a more or less deep pit 

 at the point of union of the premaxillaries and the maxillaries. The alveolar edge of 

 the upper jaw convex downwards and of the lower jaw concave upwards. The facial region 

 greatly elevated by the expansion of the upper part of the maxillaries and the lachry- 

 mals. The region is quite narrow from side to side. The orbits large, round and located 

 far back in the skull. The skull abruptly truncated posteriorly. The posterior aspect 



