Tlie Use of Grypluaa in Correlation. 265 



Many of these ofEslioots from the oyster family continued to 

 progress in tlie characters named, particularly in the curving of 

 the left valve, until a stage was reached when the coiled apical 

 portion pressed against the opercular right valve.^ Thus in such 

 advanced Gryphaeas the shell would be kept closed, and consequently 

 this stage marks the limit of evolution in that direction. 

 Occasionally large adult specimens show a tendency to avoid this 

 calamity by reducing the coiling in the later stages (Fig. 6). 



It is only necessary here to refer briefly to the interesting 

 biological aspects of the evolution of gryphaean lineages. Although 

 it may be conceded that the earliest stages in such a lineage may have 

 been of some advantage to the animal, it is obviously difficult to 

 account for evolution in a direction that inevitably leads to the 

 extinction of the lineage, if evolution is the result of the operation 

 of natinral selection. Dr. W. D. Lang has taken a different view of 

 the relative importaiTce of natural selection, and has suggested that 

 these lineages became extmct through the secretion of a superfluity 

 of calcium carbonate.^ 



Notes on Species. 



Ostrcea liassica Strickland. 



This name conveniently covers the typical oysters of the base of 

 the Lias. A flat form, usually attached by the whole surface, or 

 nearly the whole surface, of the left valve. Commonly about 1 inch 

 to H inches in length. 



These shells are most abundant in the Ostrcea Beds at the base 

 of the Lias, and are also found in the overlying flanorhe and 

 Caloceras sub-zones. In the latter, the oysters are not infrequently 

 moulded on the shells of ammonites. Oysters somewhat similar 

 to this species, frequently of larger size and less regular form, are 

 also found in several higher zones of the Lower Lias. 



Some small specimens of 0. liassica, collected at Lavernock, 

 near Cardiff, and at Owthorpe and Banistone, near Nottingham, 

 show the initial shell or prodissoconch (Fig. 7). In these young 

 shells the umbo is prominent in the free valve and is directed 

 posteriorly, as in Ostrcea eclulis.^ 



Ostrcva irregularis Miinst. 

 0. irregularis Quenstedt. Der Jura, 1853, t. iii, L5a, h. 

 Cf. 0. suhlamellosa Dunker, Dumortier. Depots jurassiques du 

 Bassin du Rhone, pt. ii, pi. i, 10, 11, 12. 

 This name is applied to oysters of the Lower Lias that are in 

 a semi-gryphseate condition. The size of the area of attachment 

 varies, but is typically about half the total length of the shell. The 

 left (attached) valve is thickened (Fig. la). 



0. irregularis is characteristic of the liasicus sub-zone, and is 



^ As in the type of G. incurva. 



^ W. D. Lane, Cntalogun of Cretaceous Brifozoa, vol. iii, 1921, p. x. 



3 R. T. Jackson, loc. cit., jj. 312. 



