THE LAMELLIBRANCHIA 89 



convex right and flat left valve, and no byssal notch ; 

 Pterinopecten and Aviculopecten (Sil.-Carb.), with long 

 hinge and ears not sharply marked off: all these have 

 feeble hinge-teeth, and are ornamented with radial 

 costae. In Spondylus (Jur.-Rec.) the cardinal area is 

 large, especially on the right valve, which is attached ; 

 the hinge-teeth are large and curved ; the surface is 

 radially costate and spiny. Plicatula (Trias.-Rec.) is 

 flatter and without the large area. Lima (Carb. ?-Rec.) 

 is equivalve, inequilateral, toothless or nearly so, and 

 with a very feeble byssal notch. 



4. Ostracea. Inequivalve; monomyarian ; interior 

 lamellar, sometimes sub-nacreous ; hinge-line short, 

 without teeth, with thick amphidetic ligament ; without 

 ears or byssal notch ; ornament concentric-striate, or 

 radial (often very coarse). Fixed by left valve. 



These are the oysters. Their very short hinge-line 

 makes the amphidetic ligament shorter than high and 

 usually triangular in shape. The subdivision into genera 

 is unsatisfactory, as the characters that have been relied 

 upon are such as recur again and again on independent 

 lines of descent. Thus strongly costate forms have been 

 named Alectvyonia; forms with the beaks curved spirally 

 backwards, Exogyra ; those with concave right valve and 

 large overhanging left umbo, the shell being fixed only 

 in early life and breaking off by its own weight later, 

 Gvyphcea (Fig. 24, g) ; but these are probably all false or 

 polyphylctic genera, consisting of a number of species 

 belonging to different oyster-stocks which have assumed 

 similar characters by convergence. True genera consist 

 of species derived from the same immediate stock. Poly- 

 phyletic genera can be recognized by differences in the 

 ontogeny of their several species. Gryphaa in particular 

 is a series of senile (phylogerontic) forms, the change of 

 growth -direction being similar to that seen in the huge 



