20 Bulletin American Museum of Natural History. [Vol. XXXIV, 
Key to Species of Didymictis. 
A. Mb,with higher trigonid, imperfectly tubercular. Protocone of m! high angulate, 
with posterior wing weak. Two posterior accessory cusps on p4, none on ps. 
1. Length of pi-m, = 33-38, of mi. = 14mm.............. D. haydenianus. 
AA. M, tubercular with low trigonid. Protocone of m! broader with anterior and 
posterior wings subequal. A posterior accessory cusp on p; and pa. 
2a. Length of pyx-m, = 45-53 mm., mj. = 16-18 mm. . D. protenus leptomylus. 
2. Length of p;x-m, = 55-60 mm., m)_2 = 19-22 mm........... D. protenus. 
2b. Length of pi-m, = 65-70 mm., mj_. = 21-24 mm... . D. protenus lysitensis. 
3. Length of pi-m, = 70-75 mm., m;_» = 24-26 mm............ D. altidens. 
Didymictis protenus leptomylus Cope 1880. 
Didymictis leptomylus Corr 1880, Amer. Nat., Vol. XIV, p. 908; 1885, Tertiary 
Vertebrata, p. 309, pl. xxva, fig. 12; pl. xxvd, fig.6; Marruew 1901, Bull. A. M.N.H., 
Vol. XIV, p. 10. 
Type, A. M. No. 4288, lower molars (mjl., mer. & 1) recorded as from the Wind 
River Basin, Wyoming, but more probably from Big Horn Basin. 
Cope distinguished this species from D. protenus by the smaller size and 
more elongate my. In 1885 he referred to it, as a larger variety, a number 
of jaws from the Big Horn Basin intermediate in size between the type and 
D. protenus. Matthew in 1901 referred to this larger variety a number of 
upper and lower jaws and fragmentary skeletons from the lower levels of 
the Big Horn Wasatch, and pointed out certain additional distinctions in 
the teeth. Several specimens of upper and lower jaws obtained by the 
‘Museum parties of 1910-12 from the Clark Fork and Sand Coulée (red- 
banded beds) and lower levels of the Gray Bull, confirm these characters. 
All the specimens, however, except one, are larger than the type, and the 
intergradation with the typical D. protenus makes it appear that this is a 
primitive subspecies scarcely entitled to distinct specific rank. 
Distinctive characters: pi-m, = 45-53 mm., mi_» = 16 mm. (type) to 18 mm. 
Parastyle of m! less extended. 
Nos. 15856, 16071, lower jaws, and several unnumbered jaw fragments 
with upper and lower teeth from the Clark Fork beds are referable to D. 
leptomylus; Nos. 2806, 2855, upper and lower jaws with considerable parts 
of skeleton are from the lower beds of the Wasatch in the Big Horn Valley 
but their exact level is uncertain. Of the later collections, the specimens 
from the lower levels of the Gray Bull horizon are all of size approximating 
the above measurements; in the middle and upper levels the specimens are 
progressively larger and agree more nearly with D. protenus. 
