VERTEBRATE FAUNA OP THE LOUP FORK BEDS. 



35 



Inflections, 3 anterior, 1 posterior - 31 



Inflections, 3 anterior, 1 posterior 40 



Inflections, 2 anterior, 1 posterior 36 



Inflections, 1 anterior, posterior 42 



III. Protohippoid. 



Inflections, 3 anterior, 3 posterior 24 



Inflections, 1 anterior, 2 posterior 28 



Inflections, 1 anterior, posterior 26 



In all these specimens the posterior loop of the anterior lake is present, 

 and it is this circumstance that induces me to think that there may be an- 

 other small Protohippus, which is a distinct species, in which this loop is 

 always absent, and to which the name of P. parvulus is applicable. As 

 has been already remarked, superior molars of three individuals of this 

 character were obtained at the same locality as that from which the speci- 

 mens of Protohippus placidus were found. These are all old individuals^ 

 with well-worn molars, the length of the crowns in two of them being 24 

 mm. and 21 mm. resjiectively. Younger teeth of this kind I did not 

 find. Considerable light is, however, thrown on this question by some 

 teeth from Nebraska, obtained for me by Mr. Frank Hazard. They ap- 

 pear to be M. i and M. iii of one side, and M. iii of the opposite side of 

 the same individual. In M. i the structure is fully protohippoid, and 

 there is neither loop nor inflection; length 25 mm. In M. iii of the same 

 side, the protocone is barely fused, there is rudimental loop and there are 

 shallow plications 2-2; length 24 mm. In M. iii of the opposite side, 

 the arrangement is hippotheroid ; there is a rudimental loop, and 2-1 

 weak plications; length 25 mm. In two superior molars obtained by 

 myself in northeast Colorado the protocone is connected with the pai'aco- 

 nule by a narrow isthmus. There is a mere trace of the loop, and the 

 enamel borders are undulate, but not plicate. Crowns 16 and 14 mm. 



It is thus evident that it is impossible to separate the P. parvidus from 

 the P. placidus by characters of the superior molars. 



A mandible lacking half the left ramus and the coronoid and angulai- 

 portions of the right, is probably referable to this species. The right, 

 series of molars, with the crowns of three incisors, and alveoli of the 

 others and of both canines are preserved. The teeth are well worn, but 

 traces of the cusps of the incisors remain. The canine is separated by 

 a short diastema from the third incisor. The third premolar is rather 

 short. The third true molar is on the contrary elongate, and has a large 

 heel, in great contrast to the P. pachyops. The molars are much worn,, 

 but it is evident that both lobes of the metaconid are tolerably well de- 

 veloped, and that the hypoconid is larger than the paraconid. The 

 mental foramen is below the middle of the diastema. No trace of a 

 fourth premolar. The last molar is elevated posteriorly on the sloping 

 base of the ramus. 



