Structure of the Lower Jaw in Ehizodopsis and Ehizodus. 141 



several bones of Bhizodics. 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. 



Sunwiary. 



The general results of the researches briefly detailed above 

 may be summed up as follows : 



The mandible has, as far as ascertained, essentially the 

 same structure in Bhizodojms as in Rhizodus. In both, the 

 dentary element is narrow and pointed posteriorly, its upper 

 margin bears one row of small teeth, while at the symphysis 

 it is peculiarly thickened where it bears the first or anterior 

 laniary. This bone, turned upside down, has, in Rhizodoims, 

 been previously considered to be the praemaxillary ; the last- 

 named element of the skull of that fish has now, however, 

 been ascertained to be a different bone, which is quite similar 

 in form and relations to the praemaxiUa in other Crossopterygii. 



The laniary teeth behind the anterior one are borne upon 

 separate internal dentary ossicles, which, when the constituent 

 elements of the lower jaw are broken up and separated, will 

 also become disarticulated and dispersed. This is absolutely 

 proved in Rhizodus, and may be considered morally certain 

 in Rhizodojosis, though a clear view of the inner aspect of the 

 complete mandible of the latter, with the posterior laniary 

 teeth in situ, has not yet been obtained. 



