502 R. TATE ON THE LIAS ABOUT EADSTOCK. 



As these limestones are on the horizon of the brick-clays and "yellow 

 lias " of Cheltenham, and of the shales forming the base of the North 

 Cheek of Robin Hood's Bay, they cannot be correlated with the 

 higher marlstone of Yorkshire, or with the still higher marlstone of 

 Gloucestershire — circumstances which warrant the abandonment of 

 the term marlstone as applied to a division of the Lias ; and its em- 

 ployment as a lithological name is equally objectionable. 



The common organic remains are mainly identical with those cha- 

 racterizing the lower beds of the Middle Lias on the Dorset and 

 Cleveland coasts, at Cheltenham and Aston Magna in Gloucestershire, 

 at Raasay and Pabba in the Hebrides, in N.W. Germany, in Wiirt- 

 temberg, and in the E. and S.E. parts of France, where the detailed 

 distribution of the fossils has been worked out, their relation to 

 higher and lower series made clear, and a remarkable uniformity of 

 facies shown to obtain. 



Local peculiarities are to be expected; and the Radstock represen- 

 tatives of the horizon are not without them. Elsewhere in the 

 British Isles the A.-Jamesoni beds are chiefly argillaceous, and the 

 ferro- calcareous nature is coordinated with the presence of re- 

 stricted species, as in N.W. Germany where similar lithological 

 features prevail the same palaeontological peculiarity is present. 



Among the prolific forms may be mentioned Trochus Thetis, *T. 

 socconensis, *Pitonillus conicus, * Phasianella turbinate,, *Mono- 

 donta bullata, * Turbo cyelostoma, * Turbo Orion, Eucyclus Guadrya- 

 nus, * Cryptosnia expansa, *C. heliciformis, Chemnitzia Blainvillei, 

 0, undulata, Ostrea semiplicata, Pinna folium, Inoceramus ventri- 

 cosus, Limea acuticosta, *Pecten lunularis, Hinnites tumidus, Ostrea 

 obliquata, *Pholadomya ambigua, Waldheimia Waterhousei, W. in- 

 dentata, W. numismalis, Terebratula punctata, PhyncJionella rimosa, 

 M. variabilis, *Ditrypa etalense, and many of the Cephalopods. It 

 is noteworthy, as illustrating the influence of the deposition of cer- 

 tain sediments upon the existence of particular specific forms, that 

 many of the above species (marked with an asterisk) reappear in 

 the ferro-calcareons rocks of the A.-spinatus beds, or in the limestones 

 of the A.-margaritatus zone, and are absent more or less in the 

 intervening clays. Despite these facts, a cosmopolitan fauna pre- 

 vails ; and the geographical distribution of its species seems to have 

 been unaffected by any lateral change in the mineral condition of 

 the depositing sediment ; the trenchant lines occupied by the Am- 

 monite-species must therefore be explained by some other cause. 



Since the publication of a former paperf , in which I endeavoured 

 to establish a palaeontological break between the Lower and Middle 

 Lias, new materials have been gathered, including some that are 

 here appended, which prove a much greater hiatus to exist than 

 was then demonstrated. Additional species in common have been 

 admitted ; but the gain of Middle-Lias forms in the A.-Jamesoni 

 zone has considerably raised the percentage of the limiting species. 

 In all localities where a succession is traceable the same general 

 phenomena meet our view, namely a paucity of species in the A.~ 

 o.rynolus series, and a sudden accession of new forms on entering 

 t Quart. Journ. Geol. Soc. vol. xxvi. 



