Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/282

 his possession longer, perhaps, than suited his pocket. At length, however. Dr. Guuther succeeded in purchasing it at a more reasonable figure, 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 displayed instead of the interior (Pl. VIII. 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 representing the maxillary teeth, or, perhaps, the bone supporting them (P1. VIII. m, 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 preserved with the inner surface exposed. The principal tooth (Pl. VIII. 1) resembles in form the mandibular plate of Ischyodus ; but the two ridges traversing obliquely the grinding-surface are more prominent, and are coarsely notched. So far, this specimen might be considered as belonging to the Chimaeroid family — perhaps an aberrant form of Ischyodus. Fortunately, however, the anterior parts of the lower jaw are preserved in their natural position, and represent a dental 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 (Pl. 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 Cochliodus ; but as the maxillary apparatus of this genus is unknown, no comparison can be instituted in this region of the mouth. The number of its component teeth corresponds with the maxillary armature of the Chimaeroids. On referring to structural analogies, we find that the teeth in this fossil follow rather the Chimaeroid 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