528 GEOLOGICAL SURVEY OF THE TEREITOEIES. 



Bemarhs. — Professor Leidy has already observed that the structure of 

 the molars in this genus is in type the same as that of the deciduous 

 molars of Eqtius, and that hence Frotohippus represents the more primi- 

 tive condition of horse. In further confirmation of this view, I may add 

 that the proportionate size of the head and length of limbs to size of 

 body is greater than in the recent species of Uquus, resembling in these 

 points the colts of that genus. Acceleration of the growth of the body 

 and prolongation of the face, the same in the widening (fore and aft) of 

 the internal columns of the molar teeth, with retardation of the growth 

 of the lateral phalanges, would express the process of evolution of the 

 modern types of horse. 



Protohippus perditus, Leidy. 



Eepresented in the collections by the entire molar dentition of one 

 cranium ; the greater part of that of another, with incisors and canines ; 

 the four median molars of another; two superior molars, with mandible 

 and teeth of a fourth ; mandibular dentition of two others, with parts of 

 mandibles and symphyses; and isolated molars of a large number of 

 additional specimens. 



Without this material, I should have hesitated to separate the two 

 species above described as new ; as it is, I have no question that they 

 are well defined, and are not the species described by Dr. Leidy under 

 the name of MerycMppus. The two lower jaws at my disposal agree in 

 dimensions with each other and with the superior molars and with Dr. 

 Leidy's types, with which I have compared them, four of them having 

 the same extent as five of those of the two species above described. In 

 two successional superior molars little worn, one of the inner columns 

 (the anterior) is not yet united with its corresponding crescent, and the 

 Ijorders of the lakes are more plicate than in more worn examples. 



Protohippus placidus, Leidy. 



A portion of the skeleton of this species was excavated by myself 

 from the rock of the Pliocene formation, which was accompanied by two 

 teeth, characteristically those of this species, and the only ones I ob- 

 tained which are referable to it. They are readily known from their 

 small size absolutely, and it would seem relatively also. The vertebrae 

 are similar in size and proportions ; but the metatarsus is materially 

 shorter than that of P. sejunetus, and the phalanges of all the toes, and 

 especially the coffin-bones, considerably stouter. Compare measure- 

 ments with those given above. 



Measurements. 



Length of median metapodial bone 0.173 



Expanse of condyles of lateral metapodiala .042 



Length of first lateral phalanx .024 



An tero-posterior width of first lateral phalanx -^, .016 



Length of coffin-bone medially .041 



Width between angles .037 



Width of articnlar face 026 



Height of coffin-bone behind .022 



Thus both coffin-bones are larger, wider, and flatter than those of P. 

 sejunetus, a character provided for by the greater lateral distal expansion 

 of the metapodial bones. The shortness of the metapodial bone may 

 be due to the fact of its being a metacarpal ; the femoral condyles are 

 adherent to it in the matrix, and there is a proximal facet like that for 



