618 PROCEEDINGS OF THE NATIONAL lilUSEUM. vol. xxxvi 



responding tooth in Ptilodus plwatus the anterior true molar. How- 

 ever, a critical examination shows that, though resembling in general 

 form the molar next behind it, it differs from that tooth in several 

 important j)articulars, while in position and function it properly 

 belongs with the antemolar series. Like the first true molar, p* is 

 much elongated anteroposteriorly and is multituberculate, but here 

 the real resemblances between the two teeth end. P* is made up of 

 two rows of cusps of unequal length, the inner, comjDosed of nine 

 tubercles of nearly uniform size, being the longer, while the tubercles 

 of the outer row, seven in number, are very unequal, some being larger 

 and some smaller than those of the inner row. The third tubercle 

 from the front in the outer row is the largest and principal cusp of 

 the tooth; in consequence the transverse diameter of the crown is 

 greatest at this point. This and the three other larger cusps of the 

 outer row have the characteristic wrinkling of the enamel seen in the 

 anterior premolars and is almost a duplicate of the outer main cusps 

 of p^; if detached, it might readily be mistaken for one of them. 

 There is a marked difference in the character of these cusps and the 

 smooth-surfaced tubercles of the true molars. 



In contrast with p", m^ is composed of two subequal rows of tuber- 

 cles of nearly uniform size and a third less prominent and much 

 shorter inner row, which, in the present specimen and in /'. plieatus^ 

 is developed only along the posterior half of the crown; thus the 

 widest transverse diameter of this tooth is at the extreme posterior 

 end instead of across the anterior half, as in p*. The last tooth of 

 the series, m-, is wider transversely, but is much shorter than m\ 

 It has three rows of tubercles, the inner and outer rows being fused 

 into an almost undivided ridge. The cusps of the median row are 

 slightly curved and point forward. 



The lower incisors are comparatively long, slender, and modei-ately 

 curved. They are oval in section at the base, but are somewhat flat- 

 tened on the inner faces and are sublanceolate near the tips. A 

 considerable portion of the tip is completely enamel-covered, al- 

 though the enamel is thin on the posterior face and it does not con- 

 tinue to the base of the crown. When placed in their normal position 

 the lower incisors come in contact with each other only along the 

 sharp angles forming the anterior borders of their inner faces. 



The lower premolars are reduced to two in number and are greatly 

 specialized, p.( being vestigial, while p^ is the largest and most highly 

 modified tooth of the lower series. It is set obliquely in the jaw, so 

 that its fore-and-aft plane comes in direct line w^ith the cheek-tooth 

 row and parallel to that of its fellow of the opposite side ; its normal 

 position is well shown in Plate 70, fig. c. The tooth crown dij^s 

 downward at a sharp angle anteriorly, so that its highest point is on a 

 level wdth the low-crowned molars. This position brings the grooves 



