the Lower Jaw in Rhizodopsis a7id Rhizodus. 303 



gigantic ally, the RMzodus of the Scottish Lower Carboni- 

 ferous strata. I had previously observed the not uncommon 

 occurrence of detached dentigerous bones belonging to R. 

 Hibbei'ti, which had exactly the same shape as the so-called 

 prsemaxillse of Rhizodopsis, 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 Rhizo- 

 dopsis. In Rhizodus there are four sjuch additional plates : of 

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

 probably equivalent to the angular element, though, indeed, 

 occu^jying also the position of a supraangular ; 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 Rhizodopsis^ the 

 name of mfradentary may be applied. 



Several detached specimens of the dentary bone oi Rhizodus 

 in the Edinburgh Museum exhibit its inner surface, which is 

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

 called prsemaxilla, of Rhizodopsis. 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 which 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 posterior laniaries are present ; but being imbedded in 

 hard ironstone, the surface of the bone is so injured as 

 to render recognition of sutures a matter of difficulty : they 

 show, however, very clearly that these posterior laniaries 

 are implanted in a thickened ledge, somewhat nodulously 

 enlarged round the base of each, and continuing backwards 

 the symphysial thickening of the dentary proper---this ledge 

 with its teeth being totally absent in the detached den- 

 taries above alluded to. I now selected for special prepa- 

 ration two jaws, seen from the outer surface, and fortunately 

 imbedded in a rather soft laminated clay. The first of 



• The more posteriorly situated laniaries of Rhizodus occur also occa- 

 sionally double. 



