RHACHITOMOUS VERTEBRAE 597 



believes that the holospondylous centrum is formed by the fusion 

 of the three bones. Under this theory, the intercentra, using the 

 term as originally applied by Cope, must be morphologically 

 different elements from the hypocentra. If such be really the 

 case, it seems probable that all the known temnospondyl amphib- 

 ians must be excluded from ancestral relationships with the 

 reptiles, since none of them is known to possess additional elements 

 in the trunk region. Aside from the improbability that the hypo- 

 centra pleuralia, known only in the tail of one or two temno- 

 spondyls I believe, have developed into so large a bone as is the 

 axial intercentrum of Dimetrodon, the relations and structure of 

 the axial and atlantal bones in the old reptiles furnish certain 

 proof, it seems to me, that the odontoid is composed exclusively 

 of the pleurocentra, though perforated by the notochordal canal; 

 and Broili's argument falls to the ground. 



Yet more conclusive evidence — it seems to me irrefutable — 

 is furnished by two caudal vertebrae (Fig. 3, B) of an unidentified 

 amphibian from the Texas Permian. The specimen was found 

 by Mr. Paul Miller in the autumn of 1908 on the Little Wichita, 

 unassociated with other bones. The size of the vertebrae rather 

 precludes the probability of their belonging with Eryops, though 

 possibly coming from near the extremity of the tail. However 

 that may be, I doubt not that a similar structure will be found 

 to be characteristic of Eryops, since by the aid of this specimen I 

 determine a like structure in the tail of Trematops, as was indeed 

 indicated by me in my paper descriptive of that genus. ^ Very 

 probably the specimen pertains to a species of Trimerorhachis. The 

 two vertebrae composing the specimen are closely associated, with- 

 out distortion, and are uninjured, save for the loss of the greater 

 part of the chevrons, and a part of the arch of the proximal verte- 

 bra. The two arches (NN), it wiJl be seen in the figure, are wedged 

 in between their adjacent pleurocentra (PP) , resting in part upon 

 the hypocentra (HH). The first pleurocentrum does not quite 

 separate the two adjacent hypocentra below, which nearly touch 

 at their extremities. The second pleurocentrum, however, is 

 almost disklike, narrowed above and below, but separating by a 



I Journal of Geology, XVII (1909), 647, Figs. 5, 16. 



