276 Dr. R. H. Traquair on the Structure 



margin of which bears an exact resemblance to that figured 

 by Dr. Young as appertaining to the mandible, the surface of 

 the bone is more convex and marked with a stronger granu- 

 lation, the tooth-tubercles are more prominent and nearly 

 equally developed on both the ridges, which are also more 

 divergent. 



Bones of both these descriptions occur in situ. In several 

 specimens I have, by careful working-out, been able to prove 

 beyond a doubt that the reputed maxilla is in reality a part 

 of the lower jaw — an internal dentary or splenial mandibular 

 element ; while the palatal nature of the bones of the second 

 category was evident from the very first. 



The internal dentary bone (sp) is represented detached in 

 PI. IX. figs. 5, 6, 7, and 8, and in situ, seen from the inner 

 aspect, in figs. 3 & 4. It is a somewhat oblong-shaped bone, 

 rather pointed in front, though more truncated posteriorly. The 

 inferior margin is thin • the upper one shows for the anterior 

 two thirds of its length two ridges, separated by a shallow 

 groove and coalescing posteriorly. The outer of the two 

 ridges is the more prominent j nearly straight at its commence- 

 ment, its contour soon assumes a gentle upward convexity, 

 this part of the ridge displaying also several small sharpish 

 dental tubercles, which are more strongly developed in some 

 individuals than in others. The inner ridge is less prominent, 

 and, proceeding in a straighter direction, gets lower down on 

 the margin as it passes forwards ; the dental tubercles are 

 quite obsolete. The inner or oral surface of the plate pre- 

 sents below the level of the smaller ridge an oblong patch of 

 enamelled granulations, roughening the lower part of the 

 wall of the cavity of the mouth — a condition also not without 

 a parallel among recent Ganoids, a portion of the oral aspect 

 of the compound splenial bone oiAmia being also granulated. 

 On working out a specimen from the outer side (fig. 7), or 

 making a transverse seetion of it, it becomes clear that the 

 plate is longitudinally sharply folded on itself along the line 

 of the tooth-bearing ridge, the outer aspect of the reflected 

 portion showing, especially towards its lower edge, which 

 articulates with the dentary bone proper, a number of minute 

 tubercles, some of the more anteriorly placed of which con- 

 siderably resemble in some cases the tubercle-teeth of the 

 marginal ridge itself. Just behind the middle of the bone 

 the externally reflected portion passes nearly horizontally out- 

 wards, showing a rather thick edge, and is then obliquely cut 

 off posteriorly (fig. 8). 



The proper dentary portion of the lower jaw belies its 

 name, in being, like the maxilla, quite edentulous, its upper 



