1880. ] Geology and Paleontology. 383 
in the anterior foot, the fourth (fifth) being represented by a 
rudimental metacarpal. It thus differs from H/yrachyus, and allied 
genera of the Eocene, and places itself in direct association with 
the three-toed forms of the Lower Miocene. The dentition is how- 
ever that of Hyrachyus. The premolars differ from the true mo- 
lars in form, and the transverse crests of the latter are uninterrupted. 
There is a diastema, in which it differs from (Helaletes) Tapirulus. 
(See Scott, Osborn and Spier on this genus.) The inferior molars 
are like those of the rhinoceroses. The ulna and radius are dis- 
tinct. I call this genus thus characterized, 7riplopus, and the 
species T. cuditalis, with the following description : 
The interorbital region of the skull is wide and flat, and the 
sagittal crest is low. The muzzle is rather short, and the anterior 
border of the orbit marks about the middle of the first true molar. 
The posterior external crescent of the superior true molars is 
without bounding or dividing ridge, while the median ridge of the 
anterior crescent is very strong. The same is true of the confluent 
crescents of the premolars. The crests of the inferior true molars 
have strong ridges descending anteriorly from their outer extrem- 
ities. The fore-limb, especially the cubitus, is rather slender. 
Length of superior molar series, m. .055; of true molars, .030; o 
Superior diastema, .012; interorbital width, .049 ; length of hu- 
merus, .1t1; of radius, .143; of median metacarpus, .066; of me- 
dian digit, 028. The species was about the size of a fox. The 
form has a good claim to be regarded as the type ancestral to 
Hyracodon—E, D: Cope. 
_ THE STRUCTURE OF THE PERMIAN GANOCEPHALA.—Examina- 
tion of abundant material shows the correctness of my anticipa- 
tion (this Journal 1878, 633), that the vertebræ of the large ba- 
trachian Æryops, would turn out to have the structure found in 
Rhachitomus. This genuś then must be referred to the same sub- 
order as Tyimerorhachis, and probably Actinodon Gaudry, which 
will be characterized by the segmented vertebral centra.. If 
European authors are correct in stating that the vertebra of the 
Labyrinthodontia have undivided centra, the sub-order above 
mentioned, must probably retain the name of Ganocephala, with 
additional characters, 
The identification of the scapular arch in Eryops, and of the 
pelvic arch in Eryops and Cricotus, gives the following results : 
The glenoid cavity is an excavation in two coossified elements, of 
which the inferior and posterior is probably coracoid. The latter 
is then much smaller than in Reptilia and Batrachia anura, but 
resembles that of the salamanders. The scapular arch proper, re- 
sembles that of the Urodela. The pelvis is intermediate between 
that of the anurous and urodelous Batrachia. There is no obtu- 
rator foramen, and the common symphysis is deep. The humerus 
closely resembles that of the Pe/ycosauria, differing chiefly in the 
Non-enclosure of the supracondylar foramen. 
