290 I*rof. 0. C. Marsh — American Jurassic Mammals. 



inserted by a single fang, but, in most cases, each has two roots, 

 although these are nearly or quite connate. When the jaw is 

 imbedded in the matrix, and the diminutive teeth uncovered as far as 

 safety will permit, the features of one side only of the molars can 

 be determined. Thus in Figure 1, Plate VIII. the outer exposed 

 side of one lower jaw (Stylacodon) is shown, while in Figure 2, the 

 inner side of another jaw [Dryolestes) is represented. In Figures 3 

 and 4, the two sides of the same jaw are placed together, and the 

 main characters of the lower molar teeth of Dryolestes are thus made 

 evident. 



There are seven superior molars, and these have one main inner 

 cone, and three outside cusps that vary in size and proportions in 

 the different genera, 



Dktolestes and Stylacodon, 



The two genera most nearly allied in dentition are Dryolestes and 

 Stylacodon, typical examples of which are shown on Plate VIII. 

 The number of lower teeth in the best preserved specimens appears 

 to be the same in each, while the incisors, canine, and four premolars, 

 show no marked differences. In Dryolestes, the eight molars which 

 follow are all of one type, and differ but little except in size. All 

 have the inner middle cone of the crown as high as, or higher than 

 the outer main cone. In Stylacodon, the first two of these teeth 

 resemble the anterior premolars in shape, and like them show from 

 the outside double fangs. The main external cone is quite as high 

 as the opposite cusp. 



In Dryolestes, moreover, the lower jaw is comparatively short and 

 massive, deep below the molar teeth, with its lower margin strongly 

 convex. The condyle in the best-preserved specimen is concave 

 transversely, and has its lower margin nearly on a line with the 

 summits of the 'molar teeth. 



In Stylacodon, the lower jaw is long and slender, and constricted 

 in front of the coronoid process, which slopes vrell upward and 

 backward. The condyle is convex transversely, and placed con- 

 siderably above the line of the teeth. The jaw is shallow below the 

 molars, scarcely exceeding the height of the teeth themselves, while 

 the lower border in this region is nearly straight. These differences 

 may be readily seen in the two specimens shown on Plate VIII. 

 Figures 1 and 2, The mylohyoid groove is well developed in both 

 genera, and its position is essentially the same in each. 



In Dryolestes, the mental foramen is below the first premolar. 

 The dental foramen is beneath the front margin of the coronoid 

 process, and at this aperture, the mylohyoid groove begins. 



ASTHENODON. 



The genus Asthenodon, the type species of which is described 

 below, agrees with the above genera in the more important characters 

 of the lower dentition, but differs, in having the entire series of 

 teeth much more uniform in size, and but eleven teeth behind the 

 canine. The type of this species is the lower jaw shown in Plate 



