MORPHOLOGICAL REVISION IO9 



nection with a rudimentary vertebra, anterior to the first, or with the skull. 

 The posterior zygapophyses are of good size. There is a small intercentrum. 

 The second vertebra has a large, double spine, showing indications that 

 the two halves were never firmly united. The posterior edge is broad, the 

 anterior thin, and extending far forward between the halves of the spine of 

 the first vertebra. There is, proportionately, a very large intercentrum in 

 this vertebra. A facet on the posterior edge of the intercentrum and on the 

 anterior edge of the pleurocentrum, with one on the lower edge of_,the pleuro- 

 centrum, form a face for the attachment of the rib. 



B C 



Fig. 39. — T. instgnis. No. +565 Am. Mus. X ?<. 



A. First two vertebrae. From right side. 



B. Four dorsal vertebrse from right side, showing intercentra 



and pleurocentra. 



C. Neural spines of three dorsal vertebrse, showing permanent 



separadon of two halves. 



In specimen No. 4557 Am. Mus., the intercentra of the first three verte- 

 brae are divided on the mid line; the halves of the first two are widely sepa- 

 rated; in the third they are distinct but still in contact. There seems to be 

 no doubt that this is a true division and not an accidental break (fig. 36 d). 

 Distinct lateral processes are visible on the first two, probably the edges of 

 the facets described above. The separate condition of the anterior inter- 

 centra is probably a survival of the primitive, phylospondylus condition. 

 No trace of such a condition is known in other forms, but it must be remem- 

 bered that the anterior vertebrse are not well known in any other form, and 

 even in Eryops the first intercentrum is missing. In the more posterior 

 vertebrse the spines are distinctly split and have a concave upper end and 

 anterior margin, indicating the presence of a large amount of cartilage. The 

 spines are inclined sharply to the rear, each overlapping the succeeding 

 vertebra. The anterior zygapophyses are well formed, but the posterior 

 are represented by faces on the sides of the neural spine near the base. The 

 intercentra show distinct faces on the posterior edge, which, in 

 combination with the similar faces on the anterior edges of the CV Pj 



pleurocentra, form facets for the rib head. The pleurocentra V^ — yj 

 are relatively small. The intercentra form nearly half a circle ^'-r'^'^ 

 and are very thin, so that the notochord is not contracted. r-^ >n 



Trimerorhachis is the most primitive of the amphibia from K^y 



Texas in this regard. Figure 40 shows the condition of inter- g 



centra in three forms in cross-section. That of Trimerorhachis 



(a) is little more than a thin section of a tubular wall around 



the notochord; in Eryops (?) reticulatus (b) the wall of the ^~^ 



intercentrum is much thickened and the notochord was greatly fig. 40. 



contracted; in Eryops megacephalus (c) the intercentrum is 



practically a wedge, with a convex lower surface and a very small notch on 



the upper, sharp edge; the notochord must have been very nearly completely 



interrupted in many vertebrse in Eryops, though it was widely expanded 



intervertebrally. The lower surfaces of the intercentra are characteristi- 



