Trof. 0. C. Marsh — American Jurassic Mammals. 245 



largest mammal of this group hitherto discovered in the Jurassic. 

 In bulk, it was three or four times as large as Allodon laticeps, and 

 about the size of a Rat. 



The only known remains of this species are from the Atlantosaurus 

 beds of the Upper Jurassic, in Wyoming. 



Ctenacodon. 



The genus Ctenacodon was based upon lower jaws, one of which 

 is represented in Plate VII. Figure 1, and others in Figures 4, 7, 

 and 8. The single, long, pointed incisor, the four, compressed, 

 cutting premolars, and the two, minute, tubercular molars, form 

 together a peculiar dentition. The long, sharp incisor shows no 

 signs of wear whatever, and hence could not be opposed to the large 

 upper incisor of Allodon. Its position was close to its fellow, and. 

 the two evidently acted together, as indicated in Figure 9, Plate VII. 

 The four premolars form a close-set series, with their upper margins 

 on a curve, all more or less notched. Some specimens, at least, show 

 distinct marks of wear on the outer sides of the crowns, which are 

 sometimes worn to a uniform surface. The two small tuberculate 

 molars are of the Microlestes type, with a deep longitudinal groove 

 on the upper surface of the crowns. 



The entire upper dentition of Ctenacodon is not known with cer- 

 tainty, but it probably corresponded in its main features to that of 

 Allodon. A portion of the upper jaw, with typical premolars, is 

 shown in Plate VII. Figures 2 and 3. The posterior premolars, 

 especially the last two, show strong marks of attrition on the inner 

 sides of the crowns, and these were opposed to the compressed pre- 

 molars below, forming together a most effective apparatus for cutting. 



Some of the lower jaws at present referred to Ctenacodon appa- 

 rently show no signs of wear on the premolars, and as the large 

 incisor is not preserved, it is impossible to say definitely that they 

 may not pertain to Allodon. It is likewise quite probable that some 

 of the lower jaws considered as Plagiaulax may belong v/ith some 

 of the specimens now known as Bolodon. The exact correlation of 

 the two forms cannot be determined with certainty until the upper 

 ■and lower jaws are found together in position. 



Ctenacodon may be distinguished from the type of Plagiaidax (P. 

 Becldesii) in having four premolars instead of three. The summits 

 of these teeth alone are notched, and the sides smooth, not obliquely 

 grooved as in Plagiaidax. The condyle, moreover, is separated from 

 the angle of the jaw, not confluent with it. Ctenacodon, also, has 

 the angle of the jaw not only strongly inflected, but its outer margin 

 ■efflected into a wide horizontal shelf, making this one of the most 

 peculiar features of the genus. 



The vertical posterior condyle in Ctenacodon implies a strong post- 

 glenoid process, that would confine the jaw to a vertical motion. 



In Ctenacodon, the mental foramen is large, and situated below the 

 middle of the diastema. The dental foramen is under the last molar, 

 but its entrance is partially concealed by a ridge descending from the 

 base of the tooth to the inflected border of the angle. 



