436 Professor 0. C. Marsh — The Skull of Protoceras. 



and longitudinally, and is marked by two deep grooves, whicli lead 

 backward to the supra-orbital foramina in the parietals. 



The frontals, which bound the nasals behind, are large massive 

 bones, much wider than long. The suture which unites the two 

 frontals is distinct, and cuts the naso-frontal suture nearly at right 

 angles. At the lateral junction of the frontal and nasal, there is on 

 each side a low tuberosity, resembling a diminutive horn-core, and 

 these form the third pair of elevations on the skull. At the postero- 

 external angle of the frontals, above the orbits, another pair of much 

 larger protuberances is seen, and the summits of these are widely 

 expanded transversely. The upper surface of the frontals is rugose, 

 and the deep grooves already mentioned are characteristic features. 



The parietal bones are much smaller than the frontals, and are 

 separated from them b}' a distinct sigmoid suture. These bones 

 support the posterior pair of horn-cores. The general form and 

 position of these elevations on the male Protoceras skull are repre- 

 sented in the accompanying plate, 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 foss^ behind. 



The inferior portion of each fossa is formed by the squamosal, 

 which covers the lower half of the brain-case, and joins the parietal 

 above by a distinct suture, as shown in Plate XIX. The squamosal 

 sends forward a short zygomatic branch, which fits into a notch in 

 the posterior part of the malar. There is a distinct 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 tympanic, separated from it above by the post- 

 tympanic process of the squamosal, and below by an open sutux-e. 

 It is wedged in between the latter bone and the strong and elongate 

 paroccipital process of the exoccipital. 



The orbit is closed behind by a descending pi-ocess 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 Plate XIX. 



The lachrymal is bounded in front by the maxillary, above by the 

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

 lachrymal foi'amina 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 narrow occiput, surmounted by the supra-occipital, is a note- 

 worthy character. The widely expanding orbits greatly increase 

 the width of the skull in this region, and from here forward its 

 wedge-like shape is a striking feature. The basi-occipital and the 

 basisphenoid bones ai-e firmly co-ossified, the suture between them 

 being indistinct. In front of the latter bone is the parasphenoid, 



