300 Dr. 1\. II. 'J'raqiiair on the Structure of 



save just in front, where it shows a slight convexity, is set 

 with a single row of small pointed teeth of nearly uniform 

 size ; but the anterior extremity bears in addition a single 

 more or less incurved laniary tooth, much larger than the 

 others, and also more internal in its position ; the opposite 

 margin, thin and sharp, displays a gently flexuous contour. 

 Seen from the inner aspect, the anterior extremity of the bone 

 presents a conspicuous thickening, in which the large laniary 

 is socketed, and which at the dental margin passes into a 

 delicate ledge, which runs back for some distance along the 

 roots of the smaller teeth. 



This bone, whose external form has been well described by 

 Messrs. Hancock and Atthey*, was considered by them to be 

 the prcemax ilia of Bhizodopsis, being obviously distinct from 

 another well-known dentigerous bone, which is indispu- 

 tably the maxilla, and closely resembles in form the maxilla 

 of Megalichthys. To all appearance it would also seem to be 

 distinct from the mandible, the margins of which " are nearly 

 parallel," and which displays, besides a large laniary tooth in 

 front, " three or four others placed along the ramus, in a line 

 M ithin the small teeth." 



With the bones described by Messrs. Hancock and Atthey 

 as the prffimaxilla, maxilla, and mandible of Rhizodopsis^ 

 every student of carboniferous ichthyology must be familiar. 

 The interpretation of the first of these as " prfemaxilla " has 

 been accepted by the Messrs. Barkasf , and, so far as I am 

 aware, has remained hitherto unquestioned. Nevertheless 

 the accuracy of its determination as such was to me a matter 

 of doubt from the first. It is true the bone in question does 

 in some measure remind us of the elongated pra3maxilla of 

 Teleostei of the most specialized type, in which that element, 

 loosely articulated with the front of the skull, extends back- 

 wards so as to shut out the now edentulous maxilla from the 

 edge of the mouth {Perca, Gadtis, &c.). But as Rhizodopsis 

 is a Crossopterygian ga.ioid of the type possessing two dorsal 

 fins and subacutely lobate pectorals, one would naturally 

 expect that its prtemaxillary bones would resemble in form 

 and relations those of its natural allies, whether rhombiferous 

 or cycliferous, in all of which, whose cranial osteology is 

 sufficiently known, each pramaxilla is comparatively small 

 and short, firmly fixed to the front of the cranial shield, and, 

 in fact, very unlike tlie bone of Rhizodopsis which has been 



• Ann. & Mag. Nag. Hist. 1808, ser. 4, vol. i. pp. 350, 3ol. 



t 'Manual of Coal-measure Palfeontologv,' by T. P. Barkas (London, 

 1873), p. 24, pi. ii. fig. (;l ; W. .7. Barkas in*^' Monthly Review of Dental 

 Surgery.' 



