588 5. W. WILLISTON 



the hard matrix, save that the clavicles and interclavicle are seen 

 to be ver}'- slender. 



It is in the limb bones that the chief distinctive characters of 

 the genus are found, characters hitherto unknown among American 

 Permian vertebrates, characters which indicate a quick-running 

 terrestrial, or, more probably, climbing reptile. The humerus 

 (Plate I, Figs. 4, 5) is very slender and delicate, with slightly 

 expanded extremities and a somewhat curved shaft. The articular 

 head is elongate oval in shape, imperfectly separated from the 

 lateral process, which is situated much closer to the head than in 

 any other Permian reptile known to me. The bicipital fossa is rather 

 deep, and there is no distinct median process. The distal extremity 

 is very thin and flat, and only moderately expanded on the ulnar 

 side. The entepicondylar foramen is small, and is situated some 

 distance above the lower end.. On the radial side there is a small 

 ectepicondylar foramen situated close to the distal margin, formed 

 by a bridge over the end of the ectepicondylar groove; it is very 

 like the foramen of Iguana. The capitellum for the radius is 

 perfectly formed, as is also the trochlear surface for the ulna; both 

 of them are very small for the slender epipodial bones. There is 

 no more characteristic bone in the early reptiles than the humerus . 

 "Ein geiibtes Auge und ein durch Nachdenken gescharfter Bhck 

 findet in dem Humerus der Reptilien zahlreiche Momente, welche 

 von mehr oder minder systematischer Bedeutung sind, welche aber, 

 was noch wichtiger ist, zugleich ein Stiick Genealogie ablesen 

 lassen."^ Among Permian reptiles I know of none other in which 

 the length exceeds the greatest width more than two and a half 

 times; in the present species the length is three and three-fourths 

 times the greatest width, a difference not often exceeded among 

 reptiles. And, even in those reptiles with a higher index, I know 

 of none in which the shaft is proportionally more slender. This 

 extreme slenderness, together with the smoothness of the bones, 

 the absence of muscular rugosities, and the perfectly formed articu- 

 lar surfaces, points, I think, toward climbing habits, or at least 

 toward purely terrestrial habits. For comparison I have given 

 in the plate (Figs, i, 2) the most slender humerus of the American 



I Furbringer, Jenaischer Zeitschr. fiir Naturwissensch., XXXIV (1900), 555. 



