292 JProf. 0. C. Marsh — American Jurassic Mammals. 



even a stronger constriction behind the molar teeth, but the jaw is 

 deeper below the molar series, and the lower margin is convex, as 

 in Bryolestes. The molar teeth resemble those of Feraspalax, Owen, 

 but in that genus there is a less number of teeth, and other features, 

 not seen in the present specimen. 



The upper jaw of this genus has not yet been identified. 



Laodon venustus. Marsh. 



In the type specimen of this species, the inner side only of the 

 lower jaw is shown. The alveolar border is nearly straight, while 

 the lower margin is strongly convex. The anterior portion of the 

 ramus is very shallow, but little, if any, deeper than the crowns of 

 the teeth it contained. There is a well-marked mylohyoid groove, 

 which begins at the dental foramen, and extends forward and down- 

 ward, until it is lost below, directly under the second molar. The 

 angle of the jaw extends well backward, and was not inflected, 

 although somewhat thickened along the lower margin. The ptery- 

 goid fossa is deep and wide. The coronoid process was large, but 

 its exact form cannot be determined. 



The type specimen of the present species is from the Upper 

 Jurassic deposits of Wyoming. 



DlPLOCYNODONTID^. 



A third group of Jurassic Mammals is known at present from 

 three genera, which have been found only in America. The most 

 typical form, DipJocynodon, is represented on Plate IX. Figure 3, by 

 the specimen first described. This fossil indicates one of the largest 

 mammals yet found in the Jurassic. In this genus, there were at 

 least three lower incisors, directed well forward. The canine is 

 very large, elevated and trenchant, and inserted by two strong fangs. 

 Behind this, there are twelve teeth, all essentially of the same type, 

 so that, from the outer side alone, it is difficult, if not impossible, 

 to distinguish the premolars from the molars. The crowns of these 

 teeth are composed of a main external cone, with a small, elevated 

 lobe in front, and a lower one behind. This is repeated on a 

 reduced scale on the inner side, except that the posterior cusp is 

 rudimentary, or wanting. The antero-posterior faces of the crowns 

 are deeply excavated, and grooved. 



The jaw is elongate, and gently curved below. The coronoid 

 process is large and elevated. The condyle is placed very low, 

 nearly on a line with the teeth. The angle of the jaw is produced 

 into a distinct process (d), the lower margin of which bends out- 

 wards, although the process as a whole has a slight inward direction. 



An upper jaw referred to this species contains the canine and 

 eight succeeding teeth in excellent preservation. The canine is 

 very large, and has two distinct fangs. The molar teeth have one, 

 main, external cone, and two lateral cusps, which rise from a strong 

 basal ridge. On the inner side, there is one main cone, with a small 

 posterior heel. The outer face and the sides of the upper molars 

 are deeply sculptured with irregular grooves. 



