30 Batrachia of the Permiau Period of North America. |Januaty, 1 
form of the basioccipital bone. In the Trimerorhachidæ its con- 
dyle is simple and concave, somewhat as in some fishes. The Ery- 
opidæ have the two condyles characteristic of Batrachia generally. 
This difference might be esteemed as of greater than family signifi- 
cance, but it is less considerable than at first sight appears. The — 
single cotylus-like basioccipital bone of the Trimerorhachide is | 
notched above, sometimes deeply, to receive the apex of the noto- 
‘chord. A corresponding notch on the inferior edge would, if 
present, divide the articulation into two surfaces, which would 
greatly resemble the condyles of Eryops. The latter are flat and 
look partly towards each other, and are evidently separated orig- 
inally by the fisswra notochorde. | 
To the Trimerorhachidz I have referred, with certainty, only 
the genus Trimerorhachis. I have been unable to learn the : 
structure of the vertebra in the European genus Archegosaurus. — 
According to some authors they are simple, as in the Stego- — 
cephali. Specimens of this.kind, of the size of the Archegosaurus 
decheni, are found in the locality where the latter occurs, 2. by 
Saarbrücken in Alsace; while rachitomous vertebrae from the — 
same locality are of larger size, and resemble those of Eryops 
(Mus. Princeton, N. J.). | 7 
I have called attention to the structure of the vertebral column — 
of the Rachitomi from a mechanical standpoint The notochord — 
persists, the osseous elements about it in the sheath or skin, in the 
form of regular concave segments much like such segments aS _ 
are cut from the skin of an orange, z. e., parts of spheres, having — 
greater or less thickness according to the group or species. Now — 
the point of divergence of these segments is on the side of the cok i 
umn, the upper segments rising, and the lower segments expanding 
downwards. To the upper segments are attached the arches and j 
their articulation, and the lower segments are like the segments OF 
a sphere. If you take a flexible cylinder, covered with a more oF 
less resistant skin or sheath, and bend that cylinder sidewise, YOU - 
will of course find that the folds of the surface will take place 
along the line of the shortest curve, which is on the side; and, : 
as a matter of fact, you will have breaks of very much the char- 
acter of the sutures of these vertebral segments. It may not be a 
symmetrical as in the actual .animal, for organic growth is sym i 
metrical so far as not interfered with; for, when we have aie : 
1 Science, 1883, p. 276. 
