234 PEOCEEDI^^GS OF THE GEOLOGICAL SOCIETY, [March 6, 



his possession longer, perhaps, than suited his pocket. At length, 

 however. Dr. Giiuther succeeded in purchasing it at a more reason- 

 able iigure, and consigned it to Lord Enniskillen, who has entrusted 

 it to me to describe. 



The first feature that arrests attention is the peculiar form of the 

 premaxillary tooth, identical with that of Ischyodus Johnsoni, but 

 having the outer surface disj)layed instead of the interior (PI. YIII.p). 

 We learn from this that this surface was striated longitudinally, and 

 that one side of the tooth was flattened for the apposition of the fellow 

 incisor. Extending at right angles on either side of the median line, 

 two plates of bone are seen occupying the position of, and probably re- 

 presenting the maxillary teeth, or, perhaps, the bone supporting them 

 (PI. YIII. rj!, m). The base of the premaxillary tooth is lodged in a 

 shallow socket at the median line ; and a similar socket is seen for the 

 reception of the corresponding tooth of the right side. The maxillaries 

 are each one inch and a half in length ; but, being imbedded in the 

 matrix, the triturating surface is not discernible. Beneath, and in 

 advance of the right maxilla, the mandible of the same side is pre- 

 servedwith the inner surface exposed. The principal tooth (PI. VIII. 1) 

 resembles in form the mandibular plate of Ischyodus ; but the two 

 ridges traversing obliquely the grinding-surface are more promi- 

 nent, and are coarsely notched. So far, this specimen might be con- 

 sidered as belonging to the Chimseroid family — perhaps an aberrant 

 form of Iscliyodus. Fortunately, however, the anterior parts of the 

 lower jaw are preserved in their natural position, and represent a den- 

 tal apparatus unlike any thing hitherto known, except in some of the 

 Cochliodont genera of the Mountain Limestone. Immediately in front 

 of the right mandible, and attached to what (in the absence of ocular 

 demonstration to the contrary) might have been termed its symphysial 

 facet, a tritoral tooth occurs not unlike one of the genus Helodus (PI. 

 VIII. 2). It is equilateral and triangular in form, and measures 

 half an inch on each side of the triangle. The grinding-surface is 

 uneven ; and the tubercular prominences with which it is studded 

 show signs of attrition. Preceding this there is another tritor, of 

 smaller dimensions and more elongated form, having the tubercles 

 arranged in oblique rows ; these also bear evidence of wear and tear 

 (PL VIII. 3). Alongside of these teeth the corresponding pair of 

 the other side are preserved in situ. 



The dental formula here disclosed is unlike any thing we know 

 among the recent Plagiostomatous fishes. If we turn to extinct 

 forms, we find that the mandibular teeth agree in number with those 

 of CocJiliodus ; but as the maxillary aj^paratus of this genus is un- 

 known, no comparison can be instituted in this region of the mouth. 

 The number of its component teeth corresponds with the maxillary 

 armature of the Chimseroids. On referring to structural analogies, 

 we find that the teeth in this fossil follow rather the Chimasroid 

 than the Cochliodont type, inasmuch as in the latter the dentary 

 plates were horizontal, rolling round and embracing, as it were, the 

 alveolar margin of the lower jaw; whereas in the former they are 

 vertical, and have the dentary portions supported by an osseous 



