FOSSIL HORSE REMAINS FROM IDAHO — GAZIN 293 



The cheek teeth in the skull of Plesippus simplicidens are badly 

 checked and in an advanced stage of wear, with the fossettes in AP 

 nearly obliterated. In all the cheek teeth the fossettes through wear 

 are much constricted and the protocone broadly united with the 

 protoconule. The occurrence of P^ is indicated by a small alveolus. 



The dorsal and lumbar vertebrae appear slightly shorter and 

 perhaps less slender than in P. shoshonensis. Measurements given 

 for the articulated series of vertebrae of P. simplicidens in table 3 

 are relativel}^ great on account of spacing of the vertebrae in the 

 mounted skeleton. The bones of the fore limb relative to those of 

 the hind limb are larger than in articulated skeletons of Plesippus 

 shoshonensis. The proportions of the hind limb of P. simplicidens 

 compare favorably with those of the hind limb of an average-sized 

 individual of P. shoshonensis, although the third metatarsal appears 

 short and relatively less slender than in the Idaho material. The 

 scapula of the Texas form shows a broad area with a double ridge 

 between the glenoid surface and the tuber scapulae, as in the Idaho 

 form, and a similarly proportioned glenoid surface. Also, as in 

 P. shoshonensis, the ridge between the bicipital grooves on the 

 proximal end of the humerus is less outstanding than the inner and 

 outer ridges. 



The essential differences between Plesippus shoshonensis and P. 

 simplicidens lie in the somewhat longer facial and shorter cranial 

 portions in the skull of P. shoshonensis, with a sharper angle between 

 the vomer and basisphenoid, and in a more elongate back and smaller 

 fore limbs. The two occur at localities widely separated geographi- 

 cally; however, there is little to suggest any significant interval of 

 tim^e between the two forms. 



Plesippus proversus (Merriam) ^^ from the upper Etchegoin in the 

 Coalinga region of California was originally described as a species of 

 Pliohippus, but recognized by Dr. Merriam as an intermediate stage 

 between Pliohippus and Equus. The type (Univ. Calif, no. 21330) 

 is an isolated upper cheek tooth, presumably M^, and referred speci- 

 mens from the same locality include portions of several upper and 

 lower cheek teeth, and incomplete foot remains. 



The material representing this species does not justify extensive 

 comparisons, although a few characters appear significant in recog- 

 nizing the position of this species and in distinguishing between it 

 and P. shoshonensis. The teeth are of good size and show moderate 

 curvature. The pattern is essentially simple, perhaps more so than 

 is common in P. shoshonensis. The protocone is elongate in the type 

 with very little of it projecting anterior to the isthmus leading to the 

 protoconule. The outline of the protocone is more nearly a smooth 

 oval than is common in the Idaho teeth. In the foot material the 



32 Merriam, J. C, Univ. California Publ. Bull. Dept. Qeol., vol. 9, no. 18, pp. 525-534, 1916. 



