14 DINOCERATA. 



In Dinoceras, they are small and sessile, and are directed upward, and 

 somewhat outward ; in Tinoceras they are larger, in most specimens, and 

 project more horizontally, usually not beyond the apex of the nasals. Some 

 of the characteristic forms of these nasals are given above, figures 3-6, 

 page 13, and others will be found under the different species in the 

 Synopsis at the end of the volume. 



On their lateral margins, the nasal bones unite by suture with the 

 superior branch of the premaxillary, and, behind this, with the maxillaries 

 up to the point where they join the frontals. These lateral sutures 

 disappear in old animals, but are shown in the skull of Dinoceras 

 mirahile, figured in Plate IV. Between the osseous pi-otuberances, or horn- 

 cores, of the maxillaries, the nasals thicken into a transverse ridge, which 

 greatly strengthens the skull in this region. The development of this 

 ridge varies in different species. The suture between the nasals and 

 maxillaries thus appears to rise on the inner face of each maxillary 

 prominence, but the nasals do not form any essential part of these 

 elevations. From this transverse ridge, the nasals expand })osteriorly, and 

 meet the frontals by oblique sutures, converging behind to the median line. 

 At the imion with the frontals, the nasal bones are comparatively thin. 



On their under surface, the nasal bones are each excavated by a 

 broad deep groove, Avhich is separated from its fellow by a sharp median 

 ridge. These grooves extend from the anterior nasal opening back to the 

 frontal bones, and then expand into large cavities immediately in front of 

 the olfactory lobes of the brain. These olfactory chambers differ in form 

 and size in different species. This part of the skull is shown in the 

 sections represented in figures 30-33, pages 29 and 30. 



Pre-Nasal Bones. "• 



The anterior extremity of the nasal bones, in both Dinoceras and 

 Tinoceras, is formed of an osseous projection, pointing forward and 

 downward, and situated in front of and below the nasal protuberances. 

 Several specimens in the Yale Museum show that this projection is formed 



