304 On the Lower Jaw in Rhizodopsis and Rhizodus. 



these was a portion of a comparatively small jaw, 3;^ inches 

 in length, and broken across f inch behind the stump of 

 the second laniary ; and by softening the matrix with water, I 

 succeeded in completely detaching it and cleaning its inner 

 surface. The surface of the bone being here quite intact, I 

 obtained a clear proof of the fact which 1 had anticipated, viz. 

 that the second laniary tooth is attached to a separate piece of 

 bone articulated by a distinct suture to the anterior thickening 

 of the dentary, and having its outer surface in apposition 

 with the flat inner surface of the dentary behind that thicken- 

 ing. The next jaw was a larger one, measuring 14 inches in 

 length, showing three entire laniaries and the stump of a 

 fourth, the articular extremity being, however, unfortunately 

 broken off. Having covered up the outer surface of the spe- 

 cimen with a sufficient mass of Portland cement, I turned it 

 over and worked down upon it from the other side, the prepa- 

 ration thus obtained entirely corroborating the conclusions pre- 

 viously arrived at. The large teeth are seen to be borne upon 

 a thickened ledge, diminishing in strength from before back- 

 wards, the anterior part of which is the previously described 

 symphysial thickening of the dentary proper, and caiTies the 

 first great laniaiy ; the suture between that and the anterior 

 of the accessory internal dentary pieces bearing the second 

 laniary is distinctly seen ; but posteriorly the separation of the 

 others is obscured by the obstinate adherence to the bone of a 

 thin layer of the matrix, which cannot be thoroughly cleared 

 off without injuring the surface. My attention was next 

 directed to a block of the same laminated clay containing 

 several bones of Rhizodus. From this I succeeded first in 

 extracting the anterior half of an isolated dentary bone, that of 

 the right side, showing the stump of the symphysial laniary 

 with the adjoining empty socket. Then, lying about 2 inches 

 from it in the same block, I observed a piece of bone bearing a 

 large tooth, which, on being in like manner extracted entire, 

 proved to be nothing more or less than the detached accessory 

 piece carrying the second laniary of the same jaw, and would 

 have fitted perfectly on to the dentary found beside it, had 

 not the latter been a little distorted by crushing. Finally, 

 several vertical sections through another mandible led to the 

 very same result — namely, that the laniary teeth behind the 

 great anterior one are attached to bone which is quite distinct 

 from that of the dentary proper ; and as the piece to which the 

 second laniary is attached has occurred quite isolated, we may 

 very safely assume that the third and fourth had also each a 

 piece for themselves. 



