298 Bulletin American Museum of Natural History. [Vol. XVI, 



monodon. They are much smaller, and the pattern of the 

 enamel lakes simpler and more irregular. Those of the lower 

 tooth are six in number, corresponding in position to the lakes 

 in M. monodon, but less elongated, fewer in number, and less 

 regularly arranged. Grinding surface of upper molar regu- 

 larly oval, with six lakes irregularly arranged. 



No. 8329 (type). No. 8330 

 Upper molar, extreme anteroposterior diameter.. 10 mm. 



" " transverse 5.5 



" anteroposterior diam. of grinding 



surface 8 



" transverse diam. of grinding surface .5.5 

 Lower molar, anteroposterior diam. (estimated) . . 8.5 



" transverse " 4.5 



Mylagaulus (Mesogaulus) ballensis Riggs. Type, a lower jaw 

 containing three teeth, from the Deep River beds of Montana. 

 Size of M. sesquipedalis, but with only four enamel lakes and, 

 according to Mr. Riggs's drawing, a heavy band of cement 

 surrounding the grinder. Our specimens show nothing like 

 this; several have a thin layer of cement over parts of the 

 outside, but never at the grinding surface, except in a sup- 

 posed milk-tooth in which the enamel ring does not come up 

 to the grinding surface. 



Dimensions, from Riggs's Description. 



Anteroposterior diameter of grinder 9 mm. 



Greatest lateral breadth of " 4.2 



Mylagaulus laevis, n. sp. 



M. monodon Matthew, Mem. Am. Mus. Nat. Hist. I, 1901, 377. 

 Not M. monodon Cope. 



Type, front half of skull and jaw, pelvis, and other frag- 

 ments from the Loup Fork of Colorado. Smaller and less 

 robust than M. monodon, pattern of lower molar similar, with 

 seven lakes arranged in four rows. Upper molar flattened ex- 

 ternally, not regularly oval like that of M. sesquipedalis, lakes 

 longer, narrower, and lying more regularly parallel. Nasals 

 smooth. 



