On Prognathodus Güntheri , Egerton, a new Genus of Fossil Fish from the Lias of Lyme Regis

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Abstract
Representatives of the Chimæroid 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 Chimæroid Fishes,” only one specimen was known, Ischyodus Johnsoni , described by Professor Agassiz in the third volume of the ‘Poissons 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 Chimæroid genera, both recent and extinct, these are subtriangular, composed of parallel 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 premaxillay tooth of the Liassic specimen differs remarkably in these particulars. Although imperfect, it measures one inch and a half in length, and has 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

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