82 GEOLOGICAL SURVEY OF TEXAS. 



The cuUing off of this posterior loop so as to make it a circle in section 

 (a column) occurs frequently in the Protohi'ppus placidus (PI. XI, Fig. 

 5a); and Leidy figures a small circle in the Equus excelsus (Ext. Mamm. 

 Dako. Nebr., Plate XXI, Fig. 31), but the presence of two such columns 

 is unprecedented in my experience. 



I have already called attention to various characters in which this 

 species differs from E. caballus*. These are chiefly in the proportions 

 and details of the skull and limbs. I can not find any character of per- 

 manent value in the dentition; that mentioned bj Leidyt, the absence of 

 the loop of the inner enamel border at the middle emargination, not 

 being constant. It is present in all the molars of the present collection, 

 and in the premolars of the skull from San Diego, Texas. The affini- 

 ties of the E. excelsus are nearer to the asinine division than to the 

 caballus division. While the molar teeth equal in dimensions those of 

 the E. caballus, the bones of the limbs and feet are more slender. I find 

 the presence of a cuboid facet on the astragalus to be almost universal 

 in the E. excelsus, viz., in five astragali from Eock creek, Texas, and in 

 fifteen out of nineteen astragali from the Oi'egon desert. It is present 

 in the quagga in my collection, but is wanting in two horses in my 

 collection, while it is present in fourteen horses in the collection of the 

 veterinary school of the University of Pennsylvania, to which I have 

 had access through the courtesies of the authorities of that institution. 



I give the following measurements of the metapodial bones of two 

 specimens, as compared with those of the horse: 



Methcarpus. 



E. caballus. E. excelsus E. excelsus 

 Texas. Oregon. 



Mm. Mm. Mm. 



Length 257 251 241 



Proximal diameters . . i anteroposerior 44 32 40 



I transverse 68 ' 53 60 



Distal width 65 48 55 



Metatarsus. 



Length 297 275 297 



Proximal diameters . . j anteroposterior 52 48 50 



1 



transverse 67 51 60 



Distal width 59 50 57 



The bones of the E. caballus belong to the same individual, while the 

 pertinence of the others to identical individuals is probable, but not certain. 

 The relatively greater robustness of the E. caballus is shown by the fig- 



*Proceeds. Amer. Philos. Soc, 1884, p. 11. 

 tExtinct Mammalia Dakota and Nebr., 1869, p. 266. 



