Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/281

 1. On PROGNATHODUS GUNTHERI, Edgerton, a new Genus of Fossil Fish from the Lias of Lyme Regis. By Sir Philip de M. Grey Egerton, Bart., M.P., F.R.S., F.G.S.

[Plate VIII.]

Representatives of the Chimaeroid family have hitherto been of very rare occurrence in the Lias. In 1847, when I communicated a paper to the Geological Society " On the Nomenclature of the Fossil Chimaeroid Fishes," only one specimen was known, Ischyodus Johnsoni, described by Professor Agassiz in the third volume of the ' Poissous Fossiles,' p. 344, and figured on pl. 40 c. fig. 22. Although included in the tabular arrangement appended to that paper, I nevertheless entertained much doubt as to the real position of this species ; it differed so much from the other members of the family in the characters of the premaxillary teeth.

In the Chimaeroid genera, both recent and extinct, these are subtriangular, composed of parallel columns of coarse dentinal matter arising from a basal pulp-cavity and lodged longitudinally in cavities of a plate of softer material, so that the unequal wear of the two substances serves to maintain notched cutting-edges at the extremities. These multiple denticles are coated externally and internally with a superficial layer of a harder material resembling ganoine. The premaxillary tooth of the Liassic specimen differs remarkably in these particulars. Although imperfect, it measures one inch and a half in length, and has a uniform breadth of half an inch. So far from being subtriangular, it resembles somewhat the incisor tooth of a gigantic Rodent, except that the inner facies is concave, and consequently the transverse section is crescentic. The profile of the tooth describes a gentle curve from the base to the extremity. The interior is composed of a homogeneous tissue of coarse tubular dentine, enveloped all round with a casing of harder material. The wearing away of these substances produces a sharp chisel-shaped edge without notches or serrations. The maxillary plate is also unlike that of any of the fossil Chimaeroids, having the triturating area composed of one broad convex layer of dentine, instead of three molar surfaces as in Edaphodon and Chimaera, or four, as in Ischyodus. In this respect it has more resemblance to Callorhynchus. The mandibular plates exhibit the same peculiarity of having the triturating surfaces continuous instead of being interrupted by bony septa (as is the case in the other members of this family). All these teeth have those exposed surfaces which are not subject to attrition in mastication invested with a thin layer of ganoine, corresponding, in this respect, with the typical Chimaeroids.

After waiting patiently more than twenty years for further materials to elucidate this aberrant form, a specimen has come into my hands, through the good offices of Lord Enniskillen and Dr. Gunther, which throws some light upon this subject. The specimen has been for some time in the possession of a man named More, at Lyme Regis, who, fancying he had found a fossil Bat, set so extravagant a price upon it that he had the satisfaction of retaining it in

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