11 



which I know to be common to the known genera. Some of them possess 

 two occipital condyles. For the purpose of avoiding the multiplication of 

 synonymes, I employ Prof. Owen's name. 



Vertebras consisting of centra and intercentra, the former not extending 

 to the base of the vertebra, the latter not rising to the neural canal. The 

 centrum consisting of two parts distinct from the superior neural arch ; 

 viz., a lateral piece (pleurocentrum), on each side. Atlas consisting of 

 separate segments, the superior of which are not united above the neural 

 canal, and the inferior (intercentrum) divided on the middle line, into two 

 segments. 



Genera. A. Basioccipital bone without condyles : Trimerorhachis 

 Cope ; Archegosaurus Meyer. A. A. Basioccipital condyles two : Actino- 

 don Guadry ; Bachithomus Cope ; Eryops Cope. 



All the above genera have well-developed neural spines except Trime- 

 rorhachis. 



ERYOPS Cope. 



Paleontological Bulletin No. 26, p. 188. Nov. 21st, 1877. Proceedings 

 Amer. Philos. Society, 1877 (1878), p. 188. 



In the essay above cited, the cranial characters of this genus were pointed 

 out with some of those of the vertebrae. It remains to describe the other 

 parts of the skeleton. Notices of some of these have already appeared in 

 the American Naturalist for September, 1878 and May, 1880. 



The largest element of the vertebra is the intercentrum. This, which 

 occupies the entire inferior surface of the vertebra, is a segment, rep- 

 resenting the sixth part of a sphere, with a slight central vacuity. The 

 element, representative of the centrum is wedged in between the superior 

 external angles of adjacent intercentra, as in Trimerorhaclm. These, as well 

 as the intercentra, differ from those of that genus in their greater degree of 

 ossification, which is so far complete as to greatly contract the canalis chor- 

 ds dorsalis. The central elements of opposite sides do not unite on the 

 middle line below, although in contact. The neurapophysis is produced 

 downwards and outwards, terminating in the simple diapophysis, with rib 

 articulation. The inferior articular faces of the arch are two on each side, 

 one for the central element in front, and the other for the one behind it. 

 The whole is surmounted by a continuous neural spine, which is expanded 

 at the summit, in the known species. The vertebrae do not differ much in 

 different parts of the column. The cervicals are not distinguished in any 

 way from the dorsals, but their anterior intercentra have more extensive 

 costal surfaces, which give the inferior posterior border lateral angles. 

 The diapophyses of the second and third cervicals are of reduced size. 

 The neural spine of the axis is a little less elevated, and is longer anteropos- 

 teriorly than that of the third and succeeding cervicals. I do not possess 

 an entire atlas free from matrix. Attached to the axis of this specimen are 

 two elements which connected it with the skull, as they are separated from 

 it only by closely fitting fractures. The elements are lateral, and each pre-' 

 sents a semi-spherical articular face in front, and a long process with acute 

 apex at right angles to it, posteriorly. These processes lie, one on each 



