302 Dr. E,. H. Traquair on the Structure of 



front of the latter and below the dentary, may be called 

 infradentary ; and there is also some evidence of a fourth, 

 small plate on the lower margin of the jaw, separating here 

 the angular from the first infradentarj for a little distance*. 



In another specimen, compressed vertically and showing 

 the top of the head, both masillfe are seen, forming the upper 

 margin of the mouth, while, forming its lower margin, both 

 dentaries are seen on the edge of the nodule, here retaining 

 their bony substance and external sculpture. Their contour 

 proves beyond a doubt that the dentary element of the mandi- 

 ble of Rhizodopsis is undistinguishable from the bone hitherto 

 reckoned as praemaxilla, but which I have already shown 

 cannot possibly be so. The very same thing is most clearly 

 shown in a shale specimen belonging to Mr. Plant of Salford, 

 in which a vertically compressed head is seen from below ; so 

 that I have no hesitation in affirming the identity of the bones 

 in question. 



Here, however, an objection to this view may be raised. The 

 mandible of Rhizodopsis Avhen perfect, as in most of the speci- 

 mens from Fenton now before me, shows not merely one large 

 tooth in front, but two or three additional ones behind it and 

 internal to the series of small teeth, though, as stated by Messrs, 

 Hancock and Atthey, these additional larger teeth "are seldom 

 present," What has become of these in the detached dentary, 

 if such be the real nature of the reputed prgemaxilla ? 



A ready explanation of this is found in the structure of the 

 lower jaw of certain Old Red Sandstone "Dendrodonts," in which 

 the laniary teeth are not attached to the dentary bone proper, 

 but to a series of accessory " internal dentary" pieces articu- 

 lated to its inner sidef. Should this be also the case with the 

 posterior laniaries of the mandible of Rhizodopsis^ then, in 

 eases where its elements are broken up and separated, these 

 additional pieces will also get detached, and the absence of all 

 but the anterior laniary in the isolated dentary bone will thus 

 be amply accounted for. 



The material at hand not furnishing me with absolute 

 proofs of this condition in Rhizodopsis ^ I now turned to its 



* That these sutures on the outer surface of the mandible in Rhizo- 

 dopsis have not been previously observed is fully accounted for by the 

 difficulty of tracing the line of demarcation between constituent and 

 closely united osseous elements, in cases where we have to deal with a 

 granulated or otherwise ornamented external bony surface. Such lines 

 of demarcation are more easily detennined where the bones are seen 

 from the inner sm'face, or where a sharp cast in hard ironstone of that 

 inner surface has been preserved. 



t See Pander's ' Saurodipterinen, Dendrodonten, &c. des devonischen 

 Systems,' pp. 41-43, tab. x. figs. 2, 3, 4, 14,22. 



