TERKESTRIAL VERTEHRATA. 253 



In Older that the result .shall be otherwise, it will be necessary to make the 

 extreme assumption, that in Cricotus the two vertebral bodies represent hypocentrum 

 pleurale (bearing chevron), and hypocentrum, i\\v i)ic'uroccntrum having disappeared. 

 The reader can judge which of the alternatives is the more probable, the disappear- 

 ance of the insignificant hypocentrum pleurale, which only exists in distinct form in 

 the SphenosauridfB, or of the large pleurocentra which arc so Avell developed in tlif 

 "Rhachitomi, and which in the caudal region of that order are almost identical with the 

 bodies marked ce in Cricotus (Fig. 3). On this hypothesis we are led to the reductio 

 ad ahsurdiim, that the bodies which do not support the chevron bones in the caudal 

 I'cgion of Eryops (PI. I, Fig. 1 c), are not homologous with the bodies which do 

 not support the chevron bones in the caudal region of Ci'icotus (Pi. I, Fig. 2 c-5) 

 with which they are identical in position and connections. 



Explanation of Plate I. 



Figs. 1-6, two-thirds natural size; Figs. 7-10, natural size ; Fig. 11, five times nat. size. 



Fig. 1. — Eryops erythrolUiem Cope (Epicordybis dim), pro.ximal caudal vertebrr. The first intercentrum is the 

 last one without chevron bone. From the Permian of Texas. 



Fig. 2. — Cricotus hypantrifus Cope, portions of the vertebral column of one individual, ."selected from the cervical 

 (a), dorsal (i), and caudal (c), regions ; left side. From the Permian of Te.xas. 



Fig. 3. — The centrum of the anterior dorsal vertebra of 2fc, fnim front, showing Uie liypnnlruiii and liypantra- 

 pophyses. (Proceedings Amer. Philos. Soc, 1884, p. 29.) 



Fig. 4. — Another dorsal centrum without neural arch, from front ; a, from behind ; b, from above, showing facet 

 or neural arch. 



Fig. 5. — Intercentrum from cervical region, showing rib atlachnient, from front ; n, from behind ; 6, from the 

 side ; f, from above. 



Fig. 6. — An intercentrum from the caudal region ; a, from above. 



Figs. 7 and 8, dorsal intcrcentra of Cricotus heterocUtus Cope, from Illinois ; collection of W. F. E. Gurley. 



Fig. 7. — Lateral view. 



Fig. 8. — Front view of another intercentrum, the superior part broken away ; a, lateral view. 



Fig. 9. — Clepsydrops nntalis Cope, a portion of the caudal region, showing ribs and chevron bone."!, from the side > 

 fig. 9a from below, .showing one free intercentrum, out of its place (»)• From the Permian of Texas. 



Fig. 10. — Splunodon gucntheri BuUer ; from New Zealand, caudal vertebra;, lateral view ; «, an intercen- 

 trum with chevron bone from front. The reticulated portions are osseous tissue. 



Fig. 11. — SpeUrpes hilineata Green, larva, vertebra;, five times natural size, linear. 11«, Three cervical vcrtebrit, 

 including atlas and connate centrum of proatlas ; lift, a dorsal vertebra ; lie, two sacral and two anterior caudal verte- 

 hrrc ; lid, second caudal vertebra from behind. The caudal vcrtcbric show the continuity of the chevron bones. Speci- 

 men from the collection of Professor S. F. Baird. 



Litters, 



pra, proatlas, centrum. poz, postzygapophysis. 



»", intercentrum. ha, hypantrum. 



p, pleurocentrum. hap, hypanlrapophysis. 



c. centrum. ne, nolochordal foramen. 

 eh, chevron hone. jf, foramina. 



r, rib. m, gacral intercentrum. 



d, dfapophysis. ic, sacral centrum. 

 pa, parapophysis. 



