28 AMERICAN MESOZOIC MAMMALIA 



possible that such would be visible were better material at hand. The incisor, for 

 instance, is unknown in American Ctenacodon lower jaws, and later multituberculates 

 show that differences in the incisors may in some cases be the only outstanding generic 

 characters in lower jaws. As already explained, Psalodon is based on an upper jaw, 

 and tPs. marshi is doubtfully referred to the genus because it includes the only known 

 lower jaws which could belong here and because it is larger than any upper or lower 

 jaws referred to Ctenacodon. 



Dentition 



The lower dental formula is that of Ctenacodon. The incisor is a large, sharply 

 pointed tooth, flattened on the inner side. Its root extends back beneath the first molar. 

 The premolars differ somewhat in proportions, but are morphologically similar to 

 those of Ctenacodon, so far as displayed, and require no separate description. The iso- 

 lated P4 (Y.P.M. No. 13669) is almost unworn and gives the characters of this un- 

 usual tooth more clearly than any other known specimen pertaining to the Plagiau- 

 lacidae. The oblique lateral ridges are seen to be better developed than would be 

 gathered from worn material, being about as long as in the English Ctenacodon 

 {Plagiatdax) minor (Falconer). Along the base of the outer face is a row of small 

 accessory cusps. Beginning at the posterior end of the tooth, there is a single blunt 

 protuberance, then a space with faint indications of a cusp, then four cusps becoming 

 progressively larger. The most anterior cuspule is at a point about one-third of the 

 length of the tooth from its anterior edge. The line along which these cuspules is 

 arranged is oblique with respect to the crown, but was horizontal in the correct orienta- 

 tion of the jaw. These projections are already truncated on this almost unworn tooth 

 and are nearly or quite removed by wear in all the other American specimens, although 

 clearly seen in some from England. This speedy obliteration after the tooth came into 

 use explains the fact that previous investigators have overlooked this unusual, but 

 minor, character. The anterior end of P4 bears a triangular depression into which the 

 posterior end of P3 fitted. 



The first molar is preserved on two specimens, but so much worn that nothing 

 further can be ascertained than that it had six cusps as in Ctenacodon. 



Mandible 



The mandible is not well shown. It apparently was much stouter than in Ctena- 

 codon. 



Measurements 

 Measurements of the type in millimeters. 



Lengths : 



P1-M2 P1.4 Pi Po P3 P4 Ml M, 



ca.io.y ca.6.s 0.7 1.7 1.7 3.0 2.1 ca. 2.1 



