NORTH AMERICA AND THEIR VERTEBRATE FAUNA. 



173 



Vertebr(B.~It is most peculiar that in all the material there are practically 

 no reptilian vertebras of the Cotylosauria ; a single imperfect vertebra of 

 Diadectes and. a few Bolosaurus vertebra have been determined. The re- 

 mainder of the many hundred vertebrae found are Pelycosaurian. Aside 

 from numerous axis vertebra which may with certainty be referred to differ- 

 ent species of Dimetrodon, large and small, there are two which belong to the 

 short-spined Pelycosaurs. The first, No. 3441 (fig. 47, a and b), is represented 

 by a complete neural arch and by an imperfect arch with a half of the cen- 

 trum. The arch is relatively low and heavy, broader behind than in front, 

 and with a pit at the posterior end of the upper edge. The anterior end does 

 not project over the anterior edge of the vertebra, as in Dimetrodon. The 

 posterior edge is thickened and rounded above, but below terminates in a 

 deep pit between the edges, which extend out upon the posterior zygapophy- 

 ses. The anterior zygapophyses are small and almost rudimentary; the pos- 

 terior are fully formed. The transverse processes descend from the anterior 

 end of the arch. This is probably the arch of the axis of Theropleura. 



Fig. 47. — Neural arch of axis. 



a, from side, (6) posterior face, of Theropleura (?), 

 No. 3441. X %. 



c, from side, (d) posterior face, of unnamed rep- 

 tile. No. 3442. X %. 



The second arch, No. 3442 (fig. 47, c), is similar to the one just described, 

 but is more slender. The spine is more expanded antero-posteriorly, so that 

 the edges overhang the lower edges of the arch. The posterior edge is thin, 

 and, instead of ending in the pit between the posterior zygopophyses, is con- 

 tinued as a sharp ridge on the bottom of the pit for some distance. At 

 present it is impossible to assign this axis to any known form, though it 

 may well belong to some one already described. 



ScapulcB. — There are several scapulae, mostly without the coracoid ele- 

 ments. Certain of them are probalbly amphibian, but as this point is uncer- 

 tain the bones are described together for easier comparison. 



The first scapula. No. 3059, is undoubtedly that of a Pelycosaur and in 

 all probability belongs to some species of Dimetrodon. The nearest bone, in 

 general form, to this is the scapula in the American Museum in New York 

 described by the author as Dimetrodon longiramus (Publication 55, Carnegie 

 Institution of Washington, p. 56, fig. 16). It differs from the typical Dime- 

 trodon scapula in the relative shortness of the blade and the greater breadth. 

 It is almost identical with the D. longiramus, except that the cotylus is some- 

 what larger (plate 22, fig. 11). 



