UPPER CRETACEOUS MULTITUBERCULATA 113 



6.0 mm. by 2.8 mm. and has a remarkably high cusp number, 9: 10:8. The posterior 

 cusps of the outer and median rows are small as are the anterior ones of the median 

 row. In all the collections many variations are seen, mostly in rather minor details, 

 which it would serve no useful purpose to describe at length. The following figures 

 give some idea of the range : 



From the grouping of these and other figures and from the morphological varia- 

 tions one has the impression of from three to six species belonging to at most two or 

 three very closely related genera. 



The t>T3e of Selenacodon brevis, Y.P.M. No. 1 1876, is the last upper molar of a 

 large member of this group. It measures 3.3 mm. in length and 3.0 in width. The cusp 

 formula is 6 : 4 : 3 and the cusps are shaped as in the preceding tooth, or much as in 

 Ptilodontids generally. The first five cusps of the outer row are subequal, the last very 

 small. In the middle row there are only three truly crescentic cusps, the anterior one 

 being developed as a transverse ridge which runs over to the inside border anterior to 

 the inner row. The three cusps of the inner row decrease in size from front to back and 

 the last is opposite the third cusp of the median row. Comparable teeth are numerous, 

 about twenty specimens having been studied carefully. They show the usual range of 

 variation. The transverse ridge anterior to the middle and inner rows is soon cut 

 through so that in older individuals it may appear to add a cusp to the inner row. In 

 one type, Y.P.M. No. 13717, this ridge is enlarged into a true anterior cusp in the 

 innerrow, the formula being 5:4:5. A smaller type is seen in Y.P.M. No. 137 16 with 

 a formula'of 4 : 3 : 2, or perhaps 4:3:3 when unworn, with the transverse anterior ridge 

 confined, even before wear, to the middle row. Other minor variants occur. 



