$6 WATSON & Day, Notes on some Paleozoic Fishes. 



PJianeropleuron has a broad depressed head with a nearly 

 flat top and laterally placed orbits about half-way between 

 the posterior end and the snout. The top of the head is 

 covered with a shield of unornamented bones whose dis- 

 tribution is shown in the figure. This shield does not 

 cover the whole surface of the head, a pair of large un- 

 ossified areas being left in the antero-lateral regions. 

 The orbit is surrounded by a single series of bones which 

 unite with the dorsal shield posteriorly and above, but 

 separate from it in front where they support a very small 

 element directed towards the snout. This element on 

 the right side of the best skull is in the same line as, and 

 separated by only a very small interval from, another very 

 slender element which itself touches the outer end of 

 another small bone provided with a row of teeth. There 

 seems no doubt that the more lateral element is the much 

 reduced representative of the maxilla. The tooth-bearing 

 bone may be either the premaxilla or the vomer. Its 

 mode of articulation with the maxilla and the fact that 

 it lies at the level of the extreme anterior end of the 

 lower jaw suggests that it is more probably the pre- 

 maxilla. It seems to have a slight extension on the 

 palate which might be the case in either bone. The 

 operculum is a thin rounded and remarkably large bone 

 lying immediately behind the circum-orbital ring. Its 

 lower margin is connected with the sub-operculum, of 

 which we can give no account. Little is shown of the 

 lower jaw beyond the fact that it extends from the back 

 of the orbit to the end of the snout, and that it includes a 

 dentary element apparently not fused with its fellow. 

 There is a trace of a bone passing backward from the 

 postero-lateral corner of the skull roof to the top of the 

 clavicle, which is presumably a supra-claviclc. The clavicle 

 itself is a large straight bone presenting no features of 



