MORPHOLOGICAL REVISION OF THE SUBORDER. II 5 



The tibia is greatly enlarged proximally. The shaft is slender and cur\'ed and 

 the lower end is only moderately expanded. The cnemial crest is a strong ridge sepa- 

 rated from the body of the bone by a deep fossa opening on the outer side. The fossa 

 is continued into the upper side of the bone as a deep pit, which nearly divides the 

 articular face into two unequal halves ; these halves are again divided by a low troch- 

 lear ridge running fore and aft. 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. 



Th&Jibiila has not been recognized in any specimen. 



The. posterior foot is not certainly known in the genus Dimetrodon. The foot of 

 the restoration is taken from the hind foot from New Mexico, which is assumed to 

 belong to D. navajavicus (plate 27) ; numerous isolated bones of larger animals show 

 that the foot must have been quite similar to that of the smaller species. 



Restoration: In the restoration the only parts drawn without warrant 

 from actual specimens are the nitmber of phalanges and the number of 

 caitdal vertebrse. The ribs are not preserved in any collection, but they were 

 observed in the ground in their natural position in the specimen No. looi 

 University of Chicago, but were so badly rotted by gypsum that they could 

 not be preserved. The posterior foot is restored from that of the genus 

 Clepsydrops. No trace of abdominal ribs has been found in any specimen 

 of the genus ; it seems fair to assume that they were absent. The height 

 of the spines, the slant to the rear, and the sudden shortening in the sacral 

 region are all repeatedly shown in the collections. The semiupright atti- 

 tude seems warranted by the shape and position of the humeral cotylus and 

 the acetabulum ; in no case can the humerus or femur be fitted into such a 

 position that the belly would touch the ground in walking. The legs were 

 far from straight and the animal must have stood in a crouching, semierect 

 position. The carnivorous dentition and strong claws both tell of a preda- 

 tory nature that must have demanded a swifter motion than could have been 

 attained by an animal which dragged its belly in progression. The tail was 

 probabl}^ rather short ; in no specimen of the many referable to the genus 

 has a complete tail been found, but in none of these is there a vertebra of 

 the elongate, cylindrical shape characteristic of long tails. Moreover, such 

 vertebrse of appropriate size are very scarce in beds prolific of other remains 

 of the genus ; the few found may well belong to the genus Theropleura. 

 The caudals presei^ved show a rapid diminution in size, so that vertebras as 

 well formed as the first caudals, but only half the size, occur in the tail of 

 Dimetrodon gigas. On the other hand, strong chevrons occur on the tail ; 

 these usually occur in forms with long and powerful tails, but they may 

 mean only strength in a comparatively short tail. 



The semiupright form, the short tail, the well-developed articulations 

 of the long bones, and the well-formed carpus and tarsus would indicate that 

 the members of this genus were true land forms. This idea is supported 



