596 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



l")rojection of the base of the coronoid process, so that the external face 

 of the dentary bone exhibits two planes. The ramus is thus more mas- 

 sive than the one above mentioned, and probably belonged to a diiferent 

 species. A tooth found with it was referred to Diclonius pentagonus. 

 (Fig. 9.) 



The third ramus was found by myself, and is more instructive than 

 the others from the fact that the sym])hyseal portion is preserved. It 

 belongs to a distinct species, and may be Dysganus haydenianus, as above 

 remarked. One of its marked characters is the form of the coronoid pro- 

 cess. It is compressed more as in Mammals, and, as in them, its anterior 

 ridge is continued as an angle along the outer side of the alveolar border 

 for a few inches. Its posterior side is deeply excavated, and the external 

 wall is extended farther backward than the interior. The magazine is 

 not nearly so deep as in the rami above described, and therefore the 

 number of teeth in a vertical column was less. This lends countenance 

 to the view that the jaw is that of a Dysganus. The separation of the 

 magazine from the external wall of the dentary is continued far forward 

 in this species, so that the former hangs suspended from the superior 

 portion of the latter. This gives the jaw a greater lightness than in 

 those above described. The alveolar line extends but a short distance 

 behind the posterior inner margin of the coronoid process. The poste- 

 rior exterior border of this process is concave; below it, the posterior 

 margin projects in an angle, which is separated by another concavity 

 from the less prominent inferior angle. The inferior border of the den- 

 tary is straight and of nearly equal thickness. The jaw in front of the 

 magazine diminishes in both diameters, and presents a thin edentulous 

 superior margin like the diastema of the Manwialia. The terminal por- 

 tion is somewhat expanded downward and thickened inward, but is 

 edentulous and without symphyseal suture. The rami evidently were 

 united by ligament only at the symphysis. The dimensions of this jaw 

 are those of the one first described. 



The fragment of the fourth lower jaw is instructive from the fact of 

 its containing the teeth in place, and so split as to give a completely 

 mineralized section of the teeth themselves. There are four in each 

 vertical column, every second one with three. The faces of the teeth 

 are directed inward, but that of the inner row is not exposed above the 

 alveolar margin. They are concealed by the persistent roots of the old 

 internal series, of which the crowns have been worn away. These roots 

 have evidently continued to grow at their bases, as they extend down- 

 ward for almost the entire lengths of the faces of the functional series. 

 The teeth below those actually in use have hollow crowns, showing that 

 a process of nutrition j)roceeds during protrusion, since the 'functional 

 teeth are filled up with dentine round the central, branched, closed fissure. 



A consideration of the preceding facts leads to the following conclu- 

 sions :— ^Professor Owen early asserted that he believed the Dinosaiiria 



