The Use of Orypluea in Correlation. 257 



The relation of Grijphcea to Ostrcea has been noticed by many 

 workers, and it is generally accepted by palaeontologists that species 

 of Grjiphcea have evolved from Ostrcea by the shortening of the 

 period of attachment and the arching of the left valve. ^ 



Numerous workers have published accounts of these fossils, 

 and some have attempted a systematic study ^ ; one of the most 

 interesting papers, published by Jones in 1865, is illustrated by 

 plates showmg the relation of species to Gnjfjhcea incurvn.^ Never- 

 theless, much research is needed before the relations of the Liassic 

 Ostrceidfv can be properly determined and before the various species 

 can be usefully employed in detailed stratigraphical work. Dr. F. L. 

 I^itchin has been for some years collecting material with a view 

 to publishing an account of Gryphcea ; his work, however, has been 

 delayed, and he has generously encouraged me to prejDare this 

 account of some of the species from the Lower Lias. Such an 

 account is a necessary preliminary to the ])ublication of the results 

 of the work on the Lower Lias of South Wales. 



The writer desires to thank numerous friends for help in the 

 field, particularly Mr. E. Drew and Mr. W. E. Howarth, who have 

 made extensive collections at various localities. Dr. W. D. Lang, 

 Mr. L. Richardson, and Mr. J. W. Tutcher have also assisted by 

 the loan of specimens. The writer also acknowledges a grant from 

 the Royal Society Committee which has enabled him to work in 

 Somerset. 



The Lowest Zones op the Lower Lias. 



The lineage of Gryphcea incurva is reprer.ented only in the lowest 

 zones of the Lower Lias, which are commonly known as the 

 jplanorhe, anrpdata, and biicUandi zones. Recently these zones 

 have been subdivided by Mr. S. S. Buckman * and Mr. .J. W. Tatcher,^ 

 and, although some of the divisions are not found in all areas, the 

 present work has shown the necessity of subdividing the original 

 zones, particularly in Glamorgan, where these zones are unusually 

 thick. For convenience, the subdivisions will be called sub-zones 

 in this paper. 



Reference Avill be made to the following : — • 



^ See R. T. Jackson, " Phvlogenv of the PeJecypoda : The Aviculidsc and 

 their Allies " : Mem. Bast. Soc. Nat/fHsL, vol. iv, 1890, p. 317. F. L. Kitchin, 

 " Summary of Progress" : Geol. Surveii, 19] 1. A. Moriey Davies, Ati Intro- 

 duction to Palceontohgy, 1920, p. 89. 



2 See, for example, M. Hebert, " Observations sur les (Hryphees du Lias, 

 etc." : Bidl. Soc. Geol. France, ser. ii, vol. xiii, 1855, p. 213 ; and O. Terquem, 

 " Paleontologie de I'Etage Inferieur de la Formation Taasique, etc." : Bvll. 

 Soc. (Vhist, nat. de la Moselle, ser. ii, vol. v, 1855. 



^ John Jones, " On Gryphsea Incurva ami its Varieties " : Proc. Coltes. Xat. 

 Club, vol. iii, 1865, pp. 81-95. 



* S. S. Buckman, "Jurassic Chronologj' : I. Lir.s " : Quart. Journ. Geol. 

 Soc., vol. Ixxiii, 1918. p. 274. 



5 J. W. Tutcher, ibid., pp. 278-81. 



VOL. LIX. — NO. VI. 17 



