24 AMERICAN MESOZOIC MAMMALIA 



conical cusps, two internal and one external, the latter a little larger and slightly 

 posterior to the middle of the tooth. All of these cusps are marked by light radial 

 furrows when unworn. Although low, the cusps have sharp points and there is a rather 

 insignificant sharp ridge running down the anterointernal cusp and up the postero- 

 internal one, and another similarly uniting the posterointernal and external cusps. 



The cusps of P* are the same low circular cones, and there is again no definite 

 longitudinal valley, but the number of cusps is higher. There is an outer row of three 

 cusps, the middle the largest and posterior the smallest. The inner row is a little longer 

 and consists of four cusps, nearly equal in size but the anterior a little the smallest. 

 The whole inner surface of the tooth to, and perhaps beyond, the apices of the internal 

 cusps has been sharply beveled by wear and a nearly vertical plane surface formed. 

 The following tooth, P**, is closely similar save that the outer cusp row is shorter, the 

 posterior cusp being absent, and the inner slope broader, probably because of the (indi- 

 vidual?) circumstance that this tooth is a little more external in the jaw than the pre- 

 ceding one. 



M' is a larger and longer tooth than P* or P^ It differs further in the fact that it 

 has a straight median longitudinal valley and that the cusps are higher, better sepa- 

 rated, and have quadrate bases, rather than circular ones. The outer row has three 

 cusps of nearly equal size and a very small anterior cuspule. The inner row has three 

 cusps equal and opposite to those of the external row and in addition it has a well 

 developed anterior cusp nearly as large as the others. 



M^ has but two cusps in the outer row, both strongly 

 developed, the anterior one exceptionally so. The inner 

 cusps are three in number, without trace of another, of 

 nearly equal size. 



As was clearly shown by Marsh, the arrangement of 

 the molars is extraordinary. The second molar is internal 

 as well as posterior to the first, so that the outer cusp row 

 Fig. 7. Ctenacodon. Diagram of M^ continues the inner cusp row of M\ The outer row 

 of upper and lower molars in ^^ j^i ^^^ .^^^^^ ^j j^2 ^^ ^.^t fit into the lower groove, 

 occlusion. Lower molar out- i i • n- 



lines darker. ^"" hence their cusps are worn only toward the midlme 



of the respective teeth, while the inner row of M' and 

 outer of M^ fit into the lower groove in occlusion and hence are reduced to mere stumps. 

 At first sight this would appear to be an individual anomaly, but it is believed to be 

 characteristic of the genus or, indeed, of the family. No other American specimens 

 show both molars in place, but some show the maxilla incurved exactly as here. Ex- 

 actly the same placing of the molars is seen in the closely allied English Bolodon. Fur- 

 thermore, wear on the lower molars always seems to be such as would be brought about 

 by such a condition in the upper molars. Finally, as will appear below, this placing of 

 the molars offers a very apt explanation of apparent anomalies in the teeth of the Ptilo- 

 dontids, which were presumably derived from a plagiaulacid-like ancestry. It is out- 

 side the present work to attempt to explain how such a novel condition can have arisen, 

 as there is no paleontological evidence bearing directly on the matter. 



