294 Prof. 0. C. Marsh — American Jurassic Mammals. 



Mbnacodon kartjs, Marsh. 



In this species, the lower jaw is comparatively slender, and its 

 inferior border is strongly convex, longitudinally. The canine was 

 small, and directed well forward. The first three premolars are 

 separated slightly from the canine, and from each other. The three 

 following teeth, which may be regarded as true molars, are larger 

 and more elevated, and behind these was the last molar, somewhat 

 smaller in size. 



In the type specimen of Menacodon, there is no sharply defined 

 mylohyoid groove, but a shallow depression takes its place, as 

 indicated in Plate IX. Figure 6. 



In Spalacoiherium there is a well-defined mylohyoid groove. 



The unique specimen on which the present species is established 

 was found in the Upper Jurassic of Wyoming. 



TlNODONTID^. 



This family is well represented by American forms, one of 

 ■which, the type species of Tinodon, is shown on Plate IX. Figure 1. 

 Phascolotherium, Owen, appears from its dentition to be an allied 

 form, but difi"ers in several important points, and may yet be found 

 to represent a distinct family. The premolar and molar teeth have 

 nearly the same form in both genera, but in Tinodon, there is a larger 

 number of post-canine teeth. The coronoid process, also, is vertical, 

 and the angle of the jaw is not inflected. The premolars have 

 the same general shape as the molars, the crowns being composed 

 essentially of three pointed cusps, one, main, outer cone, and two, 

 smaller cusps, one in front and one behind, on the inner side. 

 There is a strong cingulum on the inner surface, which may develop 

 into an anterior lobe, or posterior heel. The mylohyoid groove is 

 distinct. The condj'le in Tinodon is rounded, and somew^hat trans- 

 verse, and is separated from the jaw by a distinct neck. 



In Tinodon bellus, the dental foramen is large (Plate IX. Fig. 1,/), 

 and looks downvrard and backward. It is placed somewhat behind 

 the anterior margin of the coronoid process, and somewhat above 

 the middle line of the ramus. The deep mylohyoid groove (g) 

 leads from this opening, forward and downward. 



Triconodontid^. 



Another family related to the one last described is represented by 

 the genus Triconodon of Owen, and by one or two American forms. 

 In this group, the premolars are unlike the molars. The latter 

 are large, and their crowns are composed of three, nearly equal, 

 trenchant cusps. The premolars are compressed and trenchant, but 

 lack the anterior cusp. There is apparently more than one genus 

 included under the specimens referred by Owen to Triconodon, but 

 more specimens will be required to separate them. 



Pkiacodon. 



One of the American forms, which appears to be generically 

 distinct from the type of Triconodon, is represented below, on 

 Plate IX. Figure 9, under the name Priacondon ferox. The type 



