i8 AMERICAN MESOZOIC MAMMALIA 



larger. Each is implanted by two subequal roots, each is swollen at the base of the 

 crown, and each presents a flat oblique surface externally and another internally, the 

 two meeting at the crest and forming a sharp anteroposterior ridge. To think of the 

 teeth as a whole as compressed is a little misleading, for the swollen base is so much 

 wider than roots or crest that the maximum width of P2 is actually greater than its 

 length and in P3 these dimensions are nearly equal. The external face continues down- 

 ward anteroexternally and forms a small projecting process outside the anterior root. 

 The crest of P3 has three notches and four small angular projections. The posterior 

 three projections correspond with three low ridges which run obliquely downward and 

 somewhat forward on the lateral faces of the tooth, becoming obsolete not far from the 

 edge. 



P4 is nearly twice as long as P3 and is implanted by two stout roots of which the 

 posterior is the larger. The width is about that of P3. The chisel-like edge, instead of 

 being inclined downward and forward at about 45° as in the two preceding teeth is 

 nearly horizontal, although the anterior portion curves downward somewhat in transi- 

 tion to the more oblique preceding crest. The serrations of the edge are six in number, 

 and the lateral ridges five. In U.S.N.M. No. 2688, which is almost unworn, there is 

 plainly an outer row of small accessory basal cusps on P4, such as is described in more 

 detail below for "^.Psalodon marski. This peculiar feature apparently occurred in all 

 plagiaulacids. 



It is customary to speak of the shearing edge of the premolars as curved, but closer 

 analysis shows that it can better be described as forming two straight lines meeting at 

 an angle. The shorter, more posterior line is horizontal and comprises most of the edge 

 of P4, while the longer one is inclined at about 45° and comprises P1-3 and the anterior 

 end of P4. The mechanical advantage of such an arrangement, working up and down 

 and also back and forth against a shorter, horizontal upper blade is apparent. 



In the type. Mi is worn and M2 absent. In Y.P.M. No. 1 1832, Mi is less worn and 

 M2 is preserved, although slightly broken. Comparison of these specimens and of all 

 the other available material, much of which is excellently preserved, shows that these 

 teeth are very unlike the earlier descriptions of them. The discrepancies, however, are 

 easily explained by the fact that these molars are very seldom preserved and even when 

 present are usually badly worn. Viewing worn materials with a hand lens and without 

 unworn teeth for comparison, the number of cusps in a single row may often appear to 

 be five or six. There is, however, no doubt at present that there never occur more than 

 three in lower molars of Ctenacodon, at least. When worn a pit appears in the dentine 

 at the apex, giving the appearance of two cusps where but one really occurred. Mi has 

 six cusps arranged in two rows of three each. The outer cusps are lower and suffer 

 most from wear. The anteroexternal cusp is about half the size of the second, and the 

 second and third are about equal. The inner cusps are wholly internal to the cusps of 

 M^ in occlusion and consequently are always less worn. The second and third cusps 

 are equal and are separated by a deep notch. The anterointernal cusp is small and is 

 poorly separated from the second, appearing to be developed on the anterior slope of 

 the latter. Moderate wear may cause it to lose its individuality, so that there may 

 appear to be only two internal cusps. 



