14 
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. 
Vertebrie 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 ; Rachithomus 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 vertebre. 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 iargest 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, asin Trimerorhachis. 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- 
d@ 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 vertebre 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 isa little less elevated, and is longer anteropos- 
teriorly than that of the third and succeeding cervicals. Ido 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 tractures. The elements are lateral, and each pre- 
sents a semi-spherical articular face in front, and a long process with acute 
apex al right angles to it, posteriorly. These processes lie, one on cach 
