22 E. L. Troxell — Amynodonts in Marsh Collection, 



A. antiquus maybe distinctly separated from the others, 

 perhaps subgenerically, by the presence of both upper 

 and lower functional first premolars, and by a marked 

 difference in the general proportions of the teeth. The 

 new species, A. erectus, is small and primitive, and in this 

 respect approaches A: antiquus, but it has lost all trace 

 of the first premolars and is of a later geological horizon. 

 A. intermedius, the largest and most progressive of the 

 species, resembles Metamynodon in the form of its canines 

 and molars and approaches it in the size of the teeth, and 

 in the stage of the premolar reduction also, where the 

 premolar series measures half the molar length. 



In progressing to the state of Metamyyiodon, an 

 undoubted lineal successor, besides the slight further 

 reduction of the premolars, we note the gradual lengthen- 

 ing of the skull behind the orbits, the widening of the 

 molar teeth, the tendency toward complicated folds on 

 the premolars, the increase in the size of the canines, the 

 closmg of the external auditory meatus below, and the 

 general crowding and concentrating of the hinder part of 

 the skull near the condyles or fulcrum, this last made 

 necessary by the enormous increase in weight of the skull 

 as a whole. 



Adaptations to Physical Environment and to Feeding. 



Most of the characters which distinguish these animals 

 from the true rhinoceroses are thought by shorn, Scott, 

 and others to be a response to the needs of a semi-aquatic 

 life. This is borne out especially by the observations on 

 (1) the posture of the naso-maxillary opening, governed 

 by the short nasals; (2) the position and form of the 

 posterior nares; (3) the high, anterior position of the 

 orbits ; (4) the broad, spreading feet and their ability to 

 fold backward; and (5) the great increase in size. 



The naso-maxillary opening, or anterior nares, which 

 depends upon and at the same time determines the form 

 of the nostrils, taken together with certain features to 

 be discussed later, suggests a prehensile, or very mobile 

 lip such as one sees in the hippopotamus and other water 

 animals. The depth and position of the posterior nares 

 seem to facilitate breathing, by making a closer connec- 

 tion between the larynx and the nasal passages when the 

 mouth is full of food or water ; and further, they prevent 

 the entrance of foreign substances, water, etc., into the 



