E. L. Troxell — Amynodonts in Marsh Collection. 29 



comparison and study. The specimen itself has been 

 improved recently by further preparation and now 

 becomes the holotype of a new species. 



The genus Amynodon is much smaller than Metamyno- 

 don, and its skull has lighter parts, as shown by the zygo- 

 matic arch, the occipital condyles, the proportions of the 

 teeth, etc. 



SUull. — The posterior nares extend forward to the 

 second molar, otherwise they resemble those of Metamy- 

 nodon in form, in the depth of the opening, and in the 

 prominence of the pterygoids. The external auditory 

 meatus is open below in this genus, as shown by A, erectus, 

 sp. nov. 



The basicranial angle (17.5°) is lower than in Metamy- 

 nodon (25°); this is considered a primitive character 

 generally, but here the decrease is partly due to crushing. 

 The premaxillary appears as a narrow strip, barely visible 

 externally; the nasals fold down one third of the distance 

 on the sides; they bear no horn rugosities such as are 

 found in Colonoceras (Marsh 1884, p. 62). The deep 

 antorbital depressions are rather broader and more open 

 than in A. intermedins, where they are abrupt. There 

 are two suborbital foramina on the right side, through 

 which the nerves and blood-vessels reached the face. The 

 supra-orbital ridge is roughened, and tubercles extend 

 over and in front of the orbits. 



Dentition. — There still remain the roots of the second 

 and third upper incisors measuring about 9 mm. in diame- 

 ter; the median incisor is broken away entirely. The 

 canine alveolus measures 19 by 12 mm. In all probability 

 this tooth was not procumbent as in ^. intermedins, but 

 was more like that of A. antiquus (Scott and Osborn 

 1883) ; its shape and position can best be' judged by the 

 lower canine, which rises and curves backward almost as 

 in ArchcBotherium and is worn in a similar manner on 

 the posterior side. From the canine to the second premo- 

 lar there is a diastema of 23 mm. 



The total measurement of the premolars, 50 mm., 

 equals half that of the true molars, 98 mm. They are 

 therefore almost as reduced as are those of A. interme- 

 dins, but that species includes its vestigial P% which is 

 absent in the new species. It is seen from the wear on 

 the lower teeth of A. antiquns also that P^ was present. 

 P^ measures 31 by 19 mm. ; the outline is well preserved 

 and the worn enamel indicates a broad shelf on the 



