Structure of the Lower Jaw in Eliizodopsis and Rhizodus. 139 



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 Ehizodopsis, I now turned to its 

 gigantic ally, the Rliizodus of the Scottish Lower Carboni- 

 ferous strata. I had previously observed the not uncommon 

 occurrence of detached dentigerous bones belonging to E. 

 Hihherti, which had exactly the same shape as the so-called 

 proemaxilloe of Ehizodopsis, and, like them, frequently bear 

 only one laniary, the large one in front. On now carefully 

 examining the exterior of several more or less perfect mandi- 

 bles, it became at once evident that the bone in question was 

 nothing more or less than the dentary element, the rest of the 

 outer surface of the jaw being formed by several additional 

 bony plates quite analogous to those occurring also in Bhizo- 

 dopsis. In Bliizodus there are four such additional plates : of 

 these the posterior one, covering up the articular region, is 

 probably equivalent to the angular element, though, indeed, 

 occupying also the position of a supra-angular ; while in front 

 of it, below the dentary, and forming the lower margin of the 

 jaw, are three others, diminishing in size from behind forwards, 

 and separated from each other by sutures passing obliquely 

 forwards and upwards, and to which, as in Bhizodopsis, the 

 name of infradentary may be applied. 



Several detached specimens of the dentary bone of Bhizodus 

 in the Edinburgh Museum exhibit its inner surface, which is 

 also conformed just as in the corresponding element, the so- 

 called prsemaxilla, of BMzodopsis. The upper margin, com- 

 paratively thin, is set with one row of small teeth ; but at the 

 symphysial extremity the bone shows^a great thickening, the 

 anterior part of which is marked by a very rough area for 

 articulation with the bone of the opposite side. In this 

 thickening is implanted the anterior great laniary, behind and 

 close to wliich is another socket, usually empty, sometimes 

 occupied by a "twin" tooth.* There are also in the Mu- 

 seum several jaws seen from the internal aspect and in which 



* The more posteriorly situated laniaries of Bhizodus occur also occasionally 

 double. 



