170 Marsh — Principal Characters of the Protoceratidw. 



The parietal bones are much smaller than the frontals, and 

 are separated from them by a distinct sigmoid suture. These 

 bones support the posterior pair of horn-cores, as shown in 

 Plate IY. The general farm and position of these elevations on 

 the male Protoceras skull are represented in the accompanying 

 plates, but they differ in each species. Behind these horn- 

 cores, there is a low sagittal crest separating the deep temporal 

 fossse. Back of the parietals is the short supra-occipital, which 

 forms a weak lambdoidal crest bounding the temporal fossae 

 behind. 



The inferior portion of each fossa is formed by the squamo- 

 sal, which covers the lower half of the brain case, and joins the 

 parietal above by a distinct suture, as shown in Plate III. The 

 squamosal sends forward a short zygomatic branch, which fits 

 into a notch in the posterior part of the malar. There is a dis- 

 tinct postglenoid process. The tympanic bone is not dilated 

 into a definite bulla, but below the auditory meatus forms a 

 short descending process. The periotic is behind the tym- 

 panic, separated from it above by the post-tympanic process of 

 the squamosal, and below by an open suture. It is wedged in 

 between the latter bone and the strong and elongate parocci- 

 pital process of the exoccipital. 



The orbit is closed behind by a descending process from the 

 frontal, which meets the upper branch of the malar. Its lower 

 border is bounded by the malar, which in front joins the 

 lachrymal above and the maxillary below, as shown in Plates 

 II and III. 



The lachrymal is bounded in front by the maxillary, above 

 by the nasal and frontal, and below by the malar and maxillary. 

 The lachrymal foramina are two in number, well within the 

 orbital border. The orbits are large, suboval in outline, and 

 widely separated from each other. Their posterior position is 

 a characteristic feature of the genus Protoceras. 



The Base of the Skull. 

 The lower surface of the male Protoceras skull is repre- 

 sented in Plate Y. The narrow occiput, surmounted by the 

 supra-occipital, is a noteworthy character. The widely expand- 

 ing orbits greatly increase the width of the skull in this region, 

 and from here forward, its wedge-like shape is a striking 

 feature. The large foramen magnum and the narrow diverg- 

 ing occipital condyles are well seen in this view. The basi- 

 occipital and the basisphenoid bones are firmly coossified, the 

 suture between them being indistinct. In front of the latter 

 bone is the parasphenoid, separated from it by a well-marked 

 suture, and passing forward above the vomers, which are here 

 distinct. The pterygoids are attached to the posterior border 

 of the palatines, and above to the alisphenoids. There is no 

 distinct alisphenoid canal. 



