15. 



Peotoiiippus sejunctus, sp. uov. 



Eepresented in my collections by a nearly complete skeleton with cra- 

 nium and entire dentition, both mandibular rami and symphysis of a 

 second; mandibles and dentition of two others, with appropriate molar 

 teeth. 



The skeleton, which I excavated with my own hands from the side of 

 a bluff, adds considerably to our knowledge of this genus of horses. The 

 side of the cranium displays a considerable depression in front of the 

 orbit, which, though not so deeply impressed as described by Doctor 

 Leidy in the known species, will refer this animal to the group regarded 

 by him as a genus under the name of. 31erychippus. That the latter is 

 distinct as a genus may be questioned, and I shall follow Doctor Leidy's 

 later conclusion in uniting them.* 



The structure of the feet in this genus, as indicated by the specimens 

 of the present species, and of the ProtoMpims pladdus, proves to be iden- 

 tical with that of Hippotherium, i. e., tridactyle, the lateral toes of re- 

 duced proportions. This is important as distinguishing the genus 

 trenchantly from JEquus, and while the union of the inner columns of 

 the superior molars distinguishes it from Hippoiherium., a form of P. per- 

 ditus is described below, in which the columns are more distinct than 

 in individuals heretofore known. 



The P. sejunctus is identical in measurements with the P. labrosus, 

 and agrees with it in the simplicity of the enamel boundaries. It is also 

 a short-crowned, but the character is not so marked as in the latter. It 

 differs strikingly in the deep and convex symphysis, and, in the only 

 specimen in which its alveolar border is preserved, in the hiatus sepa- 

 rating the inferior canine from the incisors. It exhibits, also, the small 

 and one-rooted first premolar of the P. perdltus. 



The adjacent horns of the lakes of the molars are more produced out- 

 ward than the remote ones, and the enamel borders have no plications. 

 The sections of the inner columns are oval posteriorly and subround 

 anteriorly. The wearing of the last molars indicates the full maturity 

 of the animal. The canines are separated by a considerable interval 

 from the third incisors. The inferior molars are similar, in general, to 

 those of P. lahrosus. In three individuals the last lobe of the last molar 

 is a cylindroid instead of a trough-shaped column. 



The cranium, in general form, partakes of the shorter and more ele- 

 vated outline seen in all the three-toed horses. The fore-part of the 

 nasal bones and the diastema behind the canines are short. The out- 

 line of the vertex from the nose to the sagittal crest is quite plane, while 

 the posterior part of the nasal bones, etc., are much narrowed by the 

 large facial depression at the sides. This occupies the space between 

 the nasal-bones and the molar ridge, above and below, and is bounded 

 behind by the anterior border of the orbit. In front it is open, but its 

 depression follows below the nasal-bones to the diastema. While its 

 area is strongly impressed, especially superiorly and iuferiorly, it is not 

 nearly so much so as indicated by Leidy in P. insignis and P. mirabiUs, 

 but more marked than in his figure of P. perditus. My specimens of 

 the latter are not well preserved in the region in question. 



The infraorbital foramen issues above the anterior border of the first 

 true molar and the molar ridge above its i^osterior portion. The orbit 

 is closed behind and the sagittal crest is but an angle, and originates 

 above the glenoid cavity. The inion is narrowed above and projects^ 

 backward over the upper edge of the foramen magnum. Posteriorly the 



* See Quarto report on Geological Survey of the Territories, 1873, vol. I, p. ;I22. 



