246 I'rqf. 0. C. Marsh — American Jurassic Mammals. 



In none of the specimens of Ctenacodon preserved is there any 

 trace of a mylohyoid groove. 



Ctenacodon potens, Marsh. 



A third species of Ctenacodon, much larger than C. serratus, is 

 represented by several jaws and isolated teeth, discovered since the 

 first species was described. The most important of these specimens, 

 w^hich may be taken as the type, is the right upper jaw represented 

 in Plate VII. Figures 2 and 3. The lower jaw with incisor, figured 

 on the same Plate, may also be referred to this species. A second 

 lower jaw in better preservation, but without the incisor, may like- 

 wise be included, although somewhat larger in size. 



The upper jaw above mentioned agrees in its general shape with 

 that of Allodon. It indicates a short, broad skull, with strong, 

 expanded, zygomatic arches. There is a small antorbital foramen, 

 as in Allodon. The four premolars present increase in size from 

 before backward. The first and second are of the Allodon type. 

 The last two have strong marks of attrition on the inner surface of 

 their crowns, as shown in Figure 2 of the same Plate. They differ 

 from the corresponding teeth in Allodon, in being more compressed, 

 and adapted to cutting.' There were apparently two true molars, 

 which are wanting in the present specimen, but their position and 

 size are similar to those of the same teeth in Allodon. 



The left lower jaw represented in Figures 7, 8, and 9 shows that 

 the incisor in this species was very large in size, and a most effective 

 weapon. It grew from a persistent pulp, and its massive base 

 extended back under the fourth premolar. The crown is oval in 

 outline at the margin of the jaw, somewhat more compressed above, 

 and sharply pointed at the apex. There is a shallow groove on the 

 outer surface of the lower half of the crown, and a corresponding 

 depression along the middle of its inner face. A careful examination 

 shows no signs of wear on any part of the crown. 



The premolars are separated from the incisor by a long diastema. 

 The first premolar is small, without serrations, and is placed close to 

 the second. The latter is larger, inserted by two fangs, and has the 

 summit faintly notched. The third premolar was still larger, but is 

 so much fractured in the present specimen that its form and dimen- 

 sions are uncertain. The fourth premolar is very large, notched at 

 the summit, and with its outer face showdng distinct marks of wear. 



The first true molar is small, and its crown much inclined back- 

 ward. The second true molar is wanting, but its alveoli show that 

 it was also small, and placed below the first molar. 



In this species, the series of four lower premolars is placed on a 

 curve, and acts as a single cutting blade against the compressed 

 upper premolars. This curve is completed behind by the two molars, 

 which have their crowns inclined outward. 



The second and larger lower jaw referred to this species gives 

 some additional characters. The third and fourth premolars show 



1 A somewhat similar tooth of Mierolestes is figured by Owen in Mesozoic Mammals^ 

 plate i. fig. 16, Mon. Pal. Soc. toI. xxiv. 1871. 



