Marsh — Principal Characters of the Protoceratidce. 169 



depression, which probably contained a gland. In Plate II, 

 this cavity is well shown behind the maxillary horn-cores, just 

 below the point where the superior border of the skull is 

 lowest. 



The nasal bones join the maxillaries above, and complete the 

 posterior border of the large narial opening. They are of 

 moderate length on the median line, and their free anterior 

 extremities are quite short. These bones are much expanded 

 transversely, and at their widest part articulate with the 

 lachrymals. All the sutures surrounding the nasals are dis- 

 tinct, and this is true, also, of their median suture. Their 

 upper surface is convex, both transversely and longitudinally, 

 and is marked by two deep grooves, which lead backward to 

 the supra-orbital foramina in the parietals, as shown in Plate 

 IV. 



Figure 4. — Front of skull of Protoceras celer. Marsh ; seen from the left side. 

 Figure 5. — Front of skull of Protoceras nasutus, Marsh; seen from the left. 

 Both are male skulls, and drawn one-half natural size. Miocene. 

 h, maxillary horn-core ; h\ section of same. 



The frontals, which bound the nasals behind, are large mas- 

 sive 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 eleva- 

 tions on the skull. At the postero-external angle of the 

 frontals, above the orbits, another pair of much larger protu- 

 berances is seen, and the summits of these are widely expanded 

 transversely, as shown in Plate IV. The upper surface of the 

 frontals is rugose, and the deep grooves already mentioned are 

 characteristic features. 



