258 The Use of GryjiJicea in Correlation. 



Zone. Sub-zone, 



semicostatum 



'' Agassiceras (sauzeanum). 

 sciioionianum. 



hucMandi . . . < 



gmuendense. 

 vercingetorix. 

 hucMandi. 

 rotator {rotiforme). 

 ^ Vermiceras. 



(angulata. 

 liasicus. 

 Wcehneroceras (megastoma). 

 CJohnstoni (Galoceras). 

 planorhe . . . ■< planorhe. 



WstrcBa. 



It has been found in general that flat oysters of the Ostrcea liassica 

 pattern are commonest in the lowest part of the planorhe zone, and 

 that arched or semi-gryphaeate individuals resembling Ostrcea 

 irregularis are common in the liasicus sub-zone. In the angulata 

 sub-zone the most typical forms are more gryphaeate in appearance, 

 but have generally a fairly large area of attachment and resemble 

 G. dumortieri Joly. In the lowest part of the hucMandi zone wide 

 open GrypJiceas (Figs. \c and 2a) are found in Somerset and in. 

 Glamorgan, while in the upper Vermiceras and lower rotator sub- 

 zones are very abundant forms resembling G. ohliquata Sow. The 

 very arched G. incurva Sow. is found in the higher sub-zones of the 

 hucMandi zone. 



Evolution of Gryph.ea incurva. 



Besides the above-named species of Ostrcea and Gryphjca these 

 lower zones of the Lower Lias are characterized by great numbers 

 of gryphaeate and semi-grypha?ate forms. These esiibit remarkable 

 variation, but on the whole the individuals may be considered to 

 be members of a series that includes the tlat oysters of 0. liassica 

 pattern and the closely coiled G. incurva. 



In the opinion of the writer, this represents a true genetic series 

 or lineage, and G. incurva has probably evolved through innumerable 

 intermediate stages (including G. ohliquata, G. dumortieri, and 

 0. irregidaris) from oysters of the type of 0. liassica. Indeed, it is 

 doubtful whether any better example of a lineage of fossil forms 

 could be found for dero.onstration purposes, since Gryph'xa is so 

 abundant that sufScient specimens can be obtained to illustrate 

 the most minute changes. 



In such a series various progressive characters jiiay be observed, 

 of which the following appear to be most important : — 



1. The ai-chmg of the left valve increases until it is coiled through 

 more than or\e and a half whorls. 



2. The area of attachment is progressively reduced. 



