soo! Bulletin American Museum of Natural History. [Vol. XXXIV, 
Tetraclenodon Scott, 1892.' 
Protogonia Corr, 1881 (preoccupied). 
Euprotogonia Corr, 1893. 
Type, Mioclenus floverianus Cope, from the Torrejon formation of New 
Mexico. 
In some respects, particularly in the forward position of metaconules 
and the semi-isolation of the hypocone, this genus shows rather closer rela- 
tionships with Ectocion than with Phenacodus. In the absence of any 
skeleton material of Ectocion, beyond an astragalus and a few fragments, 
it is, however, not possible to make very satisfactory comparisons. 
In his Revision of the Puerco Fauna,? Dr. Matthew recognized two 
species of the genus, 7. puercensis, the common form, and 7. minor, a 
diminutive species represented by only three specimens. There is a wide 
range among the specimens referred to T'. puercensis, not only in size but 
in minor details of cusp development. This variation is found in Phena- 
codus also, and with a large series of specimens, over 300 of Tetraclenodon, 
it is very difficult to select a set of specific characters which works out 
satisfactorily. It is now known that Tetraclenodon comes from two distinct 
horizons in the Torrejon and a study of the new material of this genus, in 
which the level of each specimen is known, may yield some definite sys- 
tematic results. — 3 
Phenacodus Cope, 1873. 
Opisthotomus Corn, 1875. 
“Trispondylus’’ Corr, 1884. 
?Hohyus Marsu, 1894 (genotype only). 
Type, P. primevus Cope, from the Wasatch* of Evanston, Wyo. 
There are now in the American Museum Collection several hundred 
specimens of this genus, representing all of the Lower Eocene horizons of 
Wyoming and New Mexico. Together they present a bewildering array 
with their great variation in size and in the lesser characters of the teeth 
which in many groups would constitute good specific differences but which 
here often appear to be only individual. The difficulty is increased by the 
fact that the group shows little or no evolution throughout its range. The 
last survivors in the Lost Cabin beds cannot be separated specifically from 
1 Proc. Acad. Nat. Sci. Phila., 1892, p. 299. 
2 Bull. A. M. N. H., 1897, p. 303. 
3 Paleont. Bull., No. 17, p. 3. 
¢ Knight formation of Veatch. 
