No. 3.] OSTEOLOGY OF MESOHIPPUS AND LEPTOMERYX. 305 



Hyracotheriiim molar : the same mode of development, we have 

 just seen, is true of the posterior pillar. The eleventh element, 

 the fold of the postero-external angle of the crown, is not prom- 

 inent until we reach Equus." (Osborn, No. 42, p. 89.) 



Wide as appears to be the interval between the molar pattern 

 of Mesohippus and that of the recent forms of Equidce, it is 

 completely bridged over by the teeth of such intermediate forms 

 as Merychippus, Protohippus, etc. 



II. The Skull. (PI. XXII., Figs. 1, 2 ; PL XXIII., Fig. 17.) 



The skull of Mesohippus has been well described and figured 

 by Leidy (No. 33), but his specimens were not very complete, 

 and we need to examine this portion of the skeleton from a 

 somewhat different point of view. 



The cranium is long, narrow, and low, the face deeper, but 

 short and tapering rapidly forward, the orbits are placed far for- 

 ward, their anterior rim being over the first molar. The cranio- 

 facial axis is straight and the occipital surface high and narrow. 



The basi-occipital is broad, stout, and strongly convex, with 

 a faintly marked median keel. The condyles are rather small, not 

 strongly projecting, and in contact in the median line below ; the 

 articular surfaces are continued forward some distance upon the 

 inferior surface of the basi-occipital ; but these surfaces are nar- 

 row and nearly flat, not concave from before backward, and not 

 having the elevated anterior borders which occur in the horse. 

 The exoccipitals are rather low and broad, and form a very 

 strong convex prominence in the median line, which lodges the 

 large vermis of the cerebellum. This prominence is not thick- 

 ened and diploetic, as in Equus, but has very thin walls. The 

 superior margin of the foramen magnum is notched in the median 

 line, and on each side of the median line there is a small pro- 

 tuberance. The paroccipital processes are long, slender, and 

 compressed. The supra-occipital is quite high and narrow, con- 

 cave in the median line, and with acute margins, forming the 

 lambdoid crest ; it extends but a little way upon the roof of the 

 cranium. The occiput as a whole is proportionately higher and 

 narrower than in the horse, and the surfaces for ligamentous 

 and muscular attachment are much less prominent and rugose. 

 The foramen magnum is narrower and more circular in outline. 



