O. G. Marsh — Description of Tertiary Artiodactyles. 271 



twenty-two in number. The first lower premolar has assumed 

 the form and function of a canine, the true lower canine being 

 quite small, and resembling the incisors. The type specimen 

 figured is from the Protoceras horizon in South Dakota. 



25. 



Thinohyus, 1875.* 

 26. 



27. 



Figure 25. — Last upper molars of Thinohyus socialis, Marsh; seen from below. 

 Figure 26. — Last upper molar of Thinohyus lentus, Marsh. 

 Figure 27. — Second lower molar of Thinohyus antiquus, Marsh; top view. 

 All the figures are natural size. 



Figure 28. — Lower teeth of Thinohyus nanus, Marsh ; top view. Natural size. 



Thinohyus nanus, sp. nov. 



The present species is the smallest of the genus yet dis- 

 covered. The specimen selected as the type is represented in 

 figure 28 above, natural size. It is a left lower jaw, with the 

 third and fourth premolars and the three molars in place. The 

 front of the jaw is not preserved, but the alveole of a large 

 canine is present, and also those of the first and second pre- 

 molars, which were evidently secant. There was apparently 

 no diastema behind the canine, and the whole jaw was unusually 

 short and robust. The type specimen is from the Miocene of 

 South Dakota, but the exact horizon is not known. 



Two other species of the present genus, from the Miocene 

 of Oregon, have already been described by the writer, and in 

 figures 25 and 26 typical specimens of both are shown. The 

 largest species is Thinohyus (Dicotyles) antiquus, from the 

 same horizon in New Jersey, and the type is shown in figure 27. 



JLeptochoerus gracilis, sp. nov. 



The genus Leptochoerus was established by Leidy in 1856, 

 the type specimen being the fragment of a small lower jaw 

 containing two molar teeth, supposed to pertain to a suilline 

 mammal. f Other remains subsequently referred to the same 

 genus threw no additional light on the affinities of the animal, 

 which up to the present time have remained in doubt. 



* This Journal, vol. ix, p. 24 8, March, 1875. 

 f Proc. Acad. Nat. Sci. Phila., p. 88, 1856. 



