68 Bulletin American Museum of Natural History. [Vol. XXXIV, 
Prolimnocyon atavus sp. nov. 
Type, No. 16816, part of lower jaw and fragments of skeleton from the Gray Bull 
horizon of the Big Horn Wasatch. Nos. 16815-8, 15165-15172, 15720 and a number 
of other specimens from the same horizon and locality, are referable to the species. 
No. 16111, a jaw fragment from the lower Sand Coulée horizon in Clark Fork Basin, 
is smaller and perhaps a primitive mutant. 
Noa, /68/6 
ra ON ae 
Rie. 57. Fig. 58. 
Fig. 57. Prolimnocyon, lower jaws, natural size, from Gray Bull beds of Big Horn Basin. 
Upper figure is No. 15168, type of P. robustus; the two lower Nos. 15166, 15172, referred to 
P. atavus, but doubtfully cospecific. 
Fig. 58. Prolimnocyon atavus, part of lower jaw with mi_s inner, superior and outer 
views, enlarged to two diameters, with outline of natural size. From the type specimen No 
16816, Gray Bull beds, Big Horn Basin, , 
The molar and premolar teeth are much asin Thinocyon. Pi has a strong 
anterior basal cusp, the anterior premolars are rather short and high; P2 has 
