98 AMPHIBIA AND PISCES OF THE PERMIAN OF NORTH AMERICA 



repeatedly described and are well known. Each vertebra consists of four 

 parts: neurocentra bearing the spine, zygapophyses, and transverse pro- 

 cess; two lateral elements, the pleurocentra; and an inferior element, the 

 intercentrum. The homology of these elements has occasioned an exhaustive 

 discussion. Without reviewing the matter, it may be stated that it is con- 

 clusively settled that the pleurocentra are the basi-dorsal elements of Gadow, 

 and that the hypocentrapleurale do not occur in Eryops* 



From the fourth to the eighth, the transverse processes grow gradually 

 longer and then diminish slowly to the eighteenth. On the first three pre- 

 sacrals the transverse processes are still as large as on the eighteenth, and 

 on the first presacral there is a small free rib. 



There is but a single sacral vertebra; Branson reports two, but this is 

 clearly a mistake. The spines and zygapophyses do not differ from those of 

 the dorsal vertebrae; the transverse process is very large, with a wide face 

 for the rib, and there is an equally large face on the intercentrum instead of 

 the usual very small one. A face on the posterior edge of the pleurocentrum 

 shares partly in the articulation of the rib. The only evidence of an approach 

 to a sacrum is, that the posterior edge of the much enlarged intercentrum 

 of the sacral vertebra is thickened and closely applied to the thickened 

 anterior edge of the Intercentrum following; they do not seem to have been 

 attached except by a heavy cartilage. 



The caudal vertebm were very few in number. The greatest number 

 preserved in any specimen is ten, but as the last of these are already quite 

 small, it is probable that there was not more than double this number. The 

 intercentrum of the first caudal is elongate, but the others are normal in 

 size. Chevron bones appear on the seventh intercentrum and continue to 

 the end. They are fused with the intercentra, perforate at the upper end, 

 and the largest, the anterior ones, are nearly two-thirds the length of the 

 spine of the vertebrae to which they belong. 



The spines of the anterior caudals terminate in a heavy knob, as do 

 those of the dorsal series, but on the fourth the spine begins to bifurcate, 

 and from this to the tenth, the last caudal known, the apices are double, 

 the two parts developed as smooth conical processes. It was upon this 

 character that Cope founded the genus and species Epicordylus erythro- 

 liticus, a mistake which he recognized and corrected so far as to regard 

 Epicordylus as a synonym of Eryops. 



The ribs of Eryops are not completely known. The first evidence of 

 rib attachment is on the second vertebra, but it is not improbable that there 

 was a rib on the first. The heads of all are broad, but not distinctly divided; 

 the tubercular portion is thickened, and was applied to the end of the trans- 

 verse process. The capitular portion was thin, not separated from the tuber- 

 cular part by a notch, and in the mid-dorsals, at least, reached downward 

 and forward to fit into a notch on the posterior edge of the intercentrum. 



The dorsal ribs are elongate and flattened in the middle portion; the 

 posterior edge is continued backward in the middle third, forming a trian- 

 gular projection over the succeeding rib and a strong protection for the 

 thorax. Ribs were present on all the presacral vertebra;, but the form of 

 the posterior ones is not known. The last vertebra carries, in one specimen, 

 a small, free rib. 



*The chief papers in this discussion are reviewed in Baur's note on Archegosaurns in the American Natu- 

 ralist for 1897, p. 875, and in Branson's Structure and Relationships of the American Labyrinthodontidje, Journ. 

 GeoL, vol. xin, 1905, p. 568. 



