We recently have studied the cranial nerve foraniina in American specimens of the oldest of the Equidae, the early Eocene Hyracotherium (eohippus). Our description will not be published in its present form since, during the summer of 1953, a party from the Anierican Museum of Natural History excavated the first undistorted eohippus skulls ever found in America. In the crushed crania we had at hand we have, however, ob-' served one detail which we found to be of general significance. The mandibular branch of the trigeminal nerve (V3) appears to have been variable in its relations to the wing of the sphenoid bone (ala temporalis sphenoidei, alisphenoid). The Hyracotherium skulls A.M.N.H.2 4832 and 4831 on their right sides contain the alisphenoid relatively well preserved. In the first specimen we could not detect any trace of perforation in the wing of the sphenoid. In this individual, consequently, the mandibular nerve must have passed outward posterior to that bone as in EquHs (fig. 1). It must have shared with the internal carotid and middle meningeal arteries the anterior section of the foramen lacerum (variously termed f. lac. anterius or medium). In the second specimen, a