Prof. 0. C. Marsh — American Jurassic Mammals. 293 



The European genus AmpMtherium may possibly belong to this 

 family, but the lower canine has only a single root, and the molars 

 appear quite different from those of the American forms. 



DOCODON. 



Another genus (Docodon) of this family may be distinguished 

 from Diplocynodon by having, in the lower jaw behind the canine, 

 eleven teeth instead of twelve. The canine has two fangs, as in the 

 latter genus, and the molar teeth correspond closely in form. The 

 symphysis is very long, and the mylohyoid groove extends forward 

 to its upper border. The type specimen of this genus is shown in 

 Plate IX. Figure 2. 



Enneodon. 



A third genus, Enneodon, described below, is represented by two 

 specimens, one of which is shown on Plate IX. Figure 4. The 

 lower jaw is comparatively short and robust, and contained only 

 nine post-canine teeth, all of the same type. 



The canine in Enneodon is large, and, as in other genera of this 

 family, is inserted by two well-separated fangs. Seen from the 

 outside, its crown resembles that of a true molar, but the anterior 

 lobe is wanting. The second premolar is larger than the first, not 

 smaller, as in the type of Diplocynodon. The premolars, although 

 of the same general form as the molars, have the surface of the 

 crown more grooved, or striate. 



A second specimen of this genus agrees with that last described 

 in the main features of its dentition, but the lower jaw is less robust. 

 The canine is also more slender, and there is a small diastema 

 behind it. The first three premolars increase in size backward, but 

 are all of similar form. The angle of the jaw is considerably below 

 the lower margin of the ramus. 



The fossils on which the two species of Enneodon are based were 

 found in the Atlantosaurus beds of the Upper Jurassic, in Wyoming. 



Spalaoothkrid^. 



The type genus of this family is Spalacotlierium of Owen, but it 

 is probable that he included more than one generic form under this 

 name, in the various specimens described. In America, one well- 

 preserved jaw has been found, which appears to indicate a distinct 

 genus (Alenacodon) , and is described below. This specimen is 

 represented on Plate IX. Figures 5 and 6, 



In the typical specimens of Spalacotherium, the premolars and 

 molar teeth are ten in number, and of the same general form. The 

 crown consists of one, main, external cone, high and pointed, and 

 two, short, inner cusps, nearly equal in size, in front of and behind 

 the main cone. The canine has two fangs, and there is little or no 

 diastema behind it. 



In Menacodon, the molars have the same general form, but there 

 appear to be but seven in the post-canine series. The crowns also 

 are shorter and more robust. The canine is small, and has two roots. 



