524 GEOLOGICAL SUEVEY OF THE TEERITOKIES. 



of the crescents. The teeth are but little curved, and the base of the 

 crown, with termination of the broad longitudinal gntters, is visible, 

 although the attrition of the teeth, especially of the inferior incisors, 

 does not indicate advanced age. 



The mandible is distinguished by the length of the diastema and the 

 flatness and shallowness of the symphysis. The permanent molars are 

 all present in the specimen, and are robust in form. Except in the first 

 and last, they are characterized by the small development of the ante- 

 rior crescent horn and posterior tubercle of the inner side of the crown. 

 The horn of anterior crescent of the first molar is well produced inward, 

 broad, and simple ; the entire tooth is narrower than the other molars 

 except the fifth and sixth. The latter is a little longer than the others, 

 and possesses a posterior crescent smaller than the others. The canines 

 issue from their alveoli very close to the third incisors 5 the two pairs 

 of first and second incisors are in a nearly transverse line, in consequence 

 of the flatness of the symphysis. The median lake is half worn-out in 

 the second incisors. 



Ifeasurements. 



Length of four premolars and one molar 0.087 



Length of crown of first premolar 008 



Length of crown of second premolar .022 



Width of crown of second premolar 019 



Length of crown of first molar 018 



Width of crown of first molar 022 



Height of crown of first molar Oil 



Length of six inferior molars 113 



Length of first inferior molar 020 



Width of first inferior molar, (medially) 009 



Length of second inferior molar 019 



Width of second inferior molar 012 



Width of symphyseal trough, (least) 015 



Depth in front of foramen mentale 016 



Expanse of two middle pairs of incisors 041 



About the size of the ass. 



This species is readily distinguished from the more common P. per- 

 ditus by the peculiar form of the symphysis, more simple molar teeth, 

 with shorter crowns, and the constantly smaller size ; four mandibular 

 teeth of the latter occupying the same space as five mandibulars in the 

 P. labrosus. The first premolars are also larger and two-rooted, those 

 of P. per ditus in three specimens before me and of P. sgunctus in one 

 example being but one-rooted. 



Protohippus sejunctus. Cope, Bull. U. S. Geol. Surv., No. 1, 1874, 15. 



Eepresented in my collections by a nearly complete skeleton, with 

 cranium 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 blufl', adds considerably to our kiiowledge 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 Dr. 

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

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

 distinct as a genus may be questioned, and I shall follow Dr. 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 thi^ Protohippus placidus, proves to be 

 identical with that of Sippotherium, i. e., tridactyle ; the lateral toes of 



* See Report on Geological Survey of the Terrs., vol. i, p. 322. 



