IV. On Chondrosteus,an extinct genus of fish allied to the sturionidæ

Abstract

Before the conclusion of his great work on Fossil Fishes, Pro­ fessor Agassiz recognized in some fragmentary remains found in the lias strata at Lyme Regis, unmistakeable evidence of the existence, at that period of the earth’s deposition, of a representative of the still extant family of the Sturgeons. To this extinct fish he assigned the name Chondrosteus. The author of the present memoir has been enabled, by the examination of numerous specimens more re­ cently acquired, to describe in some detail the external features of the fish, and the structural peculiarities of those portions of the exoand endo-skeleton which have been preserved. In the former respect the fossil differs from the recent sturgeon in having a shorter and deeper trunk, in the greater vertical expanse and wider diver­ gence of the lobes of the caudal fin, in the median position of the dorsal fin, and in the absence of dermal plates on the back, belly, and flanks. Before describing the cranial anatomy, the author points out certain homologies between the head-plates of the recent sturgeon and the epicranial bones of the teleostean fishes, more especially with reference to the parietals, mastoids and frontals ; and explains that these conclusions have resulted from the examination of the inner table of skull, where the relative position and propor­ tions of the component plates are constant, however much the outer or dermal layer may vary. The remainder of the memoir is devoted to detailed descriptions of such parts as are preserved in the several specimens; and the author concludes by stating as the result of his investigations, that Professor Agassiz was right in referring the liasic fish to the Sturionidse; that in some respects it evidenced a transitional form between the latter family and the more typical ganoids; that its food was similar to that of the existing members of the family, but that it was procured in a tranquil sea, rather than in the tumultuous waters frequented by sturgeons of the present time.