78 REVISION OF THE PELYCOSAURIA. 



but well formed. Between the lower edges of the axis and atlas is a small fragment 

 of bone that is probably the remains of an intercentrum. It is the only fragment of 

 an intercentrum preserved, though intercentra were present in all parts of the column. 

 In the vertebra posterior to the axis the bottom line becomes shortened and then 

 elongate again as in the Clepsydropidcr, but the change is very much less than in 

 that family. In the first seven vertebra as the centrum becomes shorter the anterior 

 face is more and more reverted on the under surface, and the bottom line shows a 

 tendency to develop a low, rounded keel, but this never passes the incipient stage. In 

 the original description mention is made of " the narrow reflected portion of the 

 anterior border for the capitular facet," but there is no distinct facet and it is prob- 

 able that, as in the Clepsydropidce^ the head of the rib articulated with the intercen- 

 trum in the anterior portion of the vertebral column. The spines of all are short and 

 broad antero-posteriorly ; the zygapoph)ses are well developed with nearly horizontal 

 faces. (In all of the first seven vertebrae the spines and neural arches have been 

 crushed down upon the centra, considerably reducing their apparent height.) The 

 transverse processes are inclined to the rear. 



Ribs were present on all of the vertebrse. One belonging to the fourth or fifth 

 vertebra is nearly complete ; the capitulum and tuberculum are widely separated and 

 the distal end is widened. 



The separate vertebrje are longer than the anterior seven, and the anterior artic- 

 ular face is not so much reflected on to the lower surface. Several show distinct facets 

 on the edge of the centrum for the head of the rib. To three or four of the posterior 

 vertebrje are attached masses of elongate, rod-like dermal scutes which probably cov- 

 ered the abdomen (plate i, fig. 7). 



The single perfect limb bone preserved, a radius or fibula, shows that the ends 

 were devoid of well-developed articular surfaces. A single phalange is short and 

 stout. 



Measuremi'yits. 



mm. ?nm. 



Length of skull from middle of orbit to anterior Length bottom line of : 



end 140 8 th vertebra 9 



Length bottom line of : 9th 10 



Axis 15 lo'h 10 



3d vertebra 13 nth 12 



4th 12 12 th 12 



5th 10 13th 13 



6th 9 Length of rib, incomplete, attached to fourth or 



yth 8 fifth vertebra 172 



The animal with its long, low, acuminate head, straight tooth line, lack of 

 enlarged incisors, and limbs without condyles approaches in a general way to the 

 Proferosaiiria, but the larger teeth in the middle of the series, the method of their 

 implantation, the shape of the ilium, and the character of the vertebrae, all point in 

 the direction of the more specialized Pelycosauria. It is probably ver>' close to the 

 ancestral fonn of the Pelycosatn-ia, but is itself so far specialized as to preclude the 

 hope that in the Pelycosauria will be found any approach to the inception of the 

 reptilian stem. 



There are in the American Museum of Natural History and the Uni- 

 versity of Chicago collections vertebrse that indicate the possible presence 

 of a larger species of Po/iosaurus, but they are too few and indeterminate 

 to warrant a description (plate i, figs. :9-i2). 



