1854.] EGERTON FOSSIL FISHES. 367 



Cambrian to the overlying Silurian groups ; — that there is a physical 

 break between them exactly on the horizon of the May-Hill sandstone ; 

 — and that, in exact co-ordination with that break (often distinctly 

 marked by a discordancy in the position of the beds), there is a great 

 change in the fossil species ; — thus pointing out the true physical 

 and palseontological base of a Silurian system. Professor Sedgwick 

 appended a corrected tabular arrangement of the Palaeozoic rocks*, 

 in accordance with the above ; and concluded with some remarks on 

 the nomenclature of British geology. 



May 10, 1854. 



The following communications were read : — 



1. Postscript to Palichthyologic Note, No. 4f . 

 By Sir P. de M. Grey Egerton, Bart., M.P., F.R.S., F.G.S. 



It having been shown, that, in accordance with the established rule 

 of priority in scientific nomenclature, the generic prefix Tetragono- 

 lepis is applicable solely to those species which coincide in generic 

 character with the typical species originally described by Bronn, which 

 has been proved to belong to the Pycnodont family, it becomes 

 necessary to make some provision for the remainder of the spe- 

 cies hitherto classed under that title, but which undoubtedly belong 

 to the Lepidoid family. These fishes have been distinguished from 

 those of the genus Bapedius mainly by the character of their teeth ; 

 and, although that character is shown to have been subject to occa- 

 sional variations, nevertheless on taking a comprehensive view of all 

 the Dapedioid species, it seems to have been sufficiently constant to 

 warrant the continuance of the separation of the group into the uni- 

 cuspid and bicuspid species. With this view, and with the sanction 

 of Professor Agassiz, I now propose the generic prefix J^chmodus 

 (from aixfxi], apoint, and ohovs, a tooth) for the unicuspid species, the 

 bicuspid species continuing as formerly under the genus Dapedius. 



2. Palichthyologic Notes. No. 6. On a Fossil Fisn/romthe 

 Upper Beds of the New Red Sandstone at Bromsgrove. 

 By Sir P. de M. Grey Egerton, Bart., M.P., F.R.S., F.G.S. 



[Plate XL] 



The second division of the Lepidoid family of fossil fishes, the Ga- 

 noidei homocerci, is distinguished from the first, or Ganoidei hetero- 

 cerci, by having the more or less symmetrical form of tail common to 

 the generality of existing bony fishes, and to those fossil species (with 

 rare exceptions) found in strata more recent than the Trias. 



Although this character, derived from the organization of the 



* See also Report Brit. Assoc. 1853, Transact. Sect. p. 55. 

 t Quart. Journ. Geol. Soc. vol. ix. p. 274. 



