233 



IV. " On Chondrosteus, an Extinct Genus of Fish allied to the 

 Sturionicbe." By Sir PHILIP DE MALPAS GREY EGERTON, 

 Bart., F.R.S. Received April 20, 1858. 



(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 

 exo- and 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. 



The Society then adjourned to Thursday, May 20. 



