34 GEOLOGICAL SURVEY OF TEXAS. 



in the second niol.ir, but the junction of the protocone is by a veiy nar- 

 row isthmus. The third molar resembles the second in the narrower 

 connection of the protocone with the paraconule. The lalies are isolated 

 in all three of the teeth, and the anterior presents the usual loop posteri- 

 ori}'. The adjacent enamel borders of the lakes have in addition a deep 

 inflection. The anterior border of the anterior lakes has an inflection 

 also in the first and second molars, but there is none on the posterior 

 border. The cementura is well developed. First tooth small, with an 

 external crest and internal cusj). 



Measurements. 



Mm. 



Length of series of temporary molars So 



Diameters of D. m. i \ anteroposterior 9 



( transverse 7 



Diameters of D. m. ii Unteroposterior 30 



( trail verse 17 



Diameters of D. m. ill \ anteroposterior 25 



( transverse 19 



These teeth resemble very much in their details the temporary molars 

 on which Leidy proposed his Merycliippus insignis, but are much more 

 elongate anteroposteriorly, agreeing in their measurements with' the M. 

 mirabiUs, of which the temporary molars iii and iv are also figured by 

 Leidy. The latter appear to present a later stage of wear, and therefore 

 a greater transverse diameter. 



PKOTOHIPPUS PLACiDus, Lcidy. 



Plate XI, Figures 3-8. For synonymy see above. 



Teeth from eight different localities in Donley county represent proba- 

 bly ten individuals of this species. The characters of the superior molars 

 differ much with age and wear, as already remarked. Younger animals 

 with less worn teeth represent the Hippotherium gratum of Leidy. In the 

 early stage of wear not only is the protocone distinct from the j^araconule, 

 but the adjacent enamel borders of the lakes are rather complex. Thus 

 there are, besides the loop on the posterior internal border of the anterior 

 lake, three folds of the opposed enamel lake border, but also sometimes 

 an emargination of the posterior border of the posterior lake. The num- 

 ber of emai'ginations diminishes with wear, until in old teeth all, except- 

 ing perhaps the loop, disappear. 



I give the following list of teeth, with their length and the complexity 

 of enamel which they present: 



/. Hippotheroid. 



Length Mm. 



Inflections, 3 anterior, 2 posterior 36 



Inflections, 2 anterior, 2 posterior, 49 



