marsh.] STEGOSAURUS. 187 



nearly parallel with the top of the skull. In this view the lower jaw 

 covers the teeth entirely. 



Seen from above, as shown in fig. 3, PI. XLIII, the wedge-shaped form 

 of the skull is still apparent. The only openings visible are the supra- 

 temporal fossse. The premaxillary bones are short above, but send back 

 a long process below the narial orifice. The nasal bones are A^ery large 

 and elongate. They are separated in front by the premaxillaries, and 

 behind by anterior projections from the frontal bones. The prefrontals 

 are large, and are placed between the nasals and the prominent, rugose 

 supraorbitals. The frontals are short, and externally join the post- 

 frontals. The parietals are small and closely coossified with each 

 other. 



Viewed from in front, the skull and mandible present a nearly quad- 

 rate outline (PI. XLIII, fig. 2), and the mutual relations of the facial 

 bones are well shown. In this view is seen, also, the predentary bone, 

 a characteristic feature of the mandible in this genus. The lateral 

 aspect of this bone is shown in fig. 1 of the same plate. 



The teeth in this genus are entirely confined to the maxillary and 

 dentary bones, and are not visible in any of the figures here given. 

 They are small, with compressed, fluted crowns, which are separated 

 from the roots by a more or less distinct neck. The premaxillary and 

 the predentary bones are edentulous. A typical tooth of Stegosaurus, 

 and one of an allied genus. Priconodon, are represented on PI. XLIV, 

 figs. 1 and 2. 



The present skull belongs to the type specimen of a very distinct 

 species, Stegosaurus stenops. The skull and nearly complete skeleton of 

 this specimen, with most of the dermal armor in place, were found 

 almost in the position in which the animal died. 



This reptile was much smaller than those representing the other 

 species of this genus. Its remains were found by Mr. M. P. Pelch in 

 the Atlantosaurus beds of the Upper Jurassic, in southern Colorado. 

 In this geological horizon all known American forms of Stegosaurus 

 have been discovered. 



THE BRAIN. 



Little has been known hitherto of the brain in dinosaurs, but fortu- 

 nately one specimen of Stegosaurus has the brain case well preserved 

 and apparently without distortion. Figs. 3 and 4 of PI. XLIV show 

 the form and general characters of this brain cavity. The brain of 

 this reptile was much elongated, and its most striking features were 

 the large size of the optic lobes and the small cerebral hemispheres. 

 The latter had a transverse diameter only slightly in excess of the 

 medulla. The cerebellum was quite small. The optic nerve corre- 

 sponded in size with the optic lobes. The olfactory lobes were of large 

 size. As a whole, this brain was lacertilian rather than avian. A 

 brain cast of a young alligator is given on PI. LXXVII for comparison. 



