THE LAMELLIBRANCHIA 



67 



ornament consists of coarse diverging ribs, sub-angular 

 to rounded in section, increasing in number by bifurcation 

 or by intercalation of new ribs. The whole surface has 

 a scaly look, due to the highly laminated structure of the 

 shell. The right valve is nearly smooth, except for con- 

 centric corrugations due to growth irregularities, but also 

 shows this laminated texture. In the umbonal region 

 there is an area of more irregular character corresponding 

 exactly to the area of attachment of the left valve. That 



add 



FlG. 20. OSTREA VENT1LABRUM, GOLDFUSS, OLIGOCENE. ( X .) 



Right-hand figure shows both valves seen from the right side ; in ihe left- 

 hand figure the right valve is removed, add. , Adductor impression; 

 Hg., ligament. (Original.) 



the surface of the foreign body should thus " show 

 through " a thick pair of valves is at first sight puzzling, 

 especially as no trace of it is seen on the interior of either 

 valve. The explanation is that the oyster fixed itself 

 when very young, both valves of the very thin shell, as 

 well as the animal's body between, being moulded up and 

 down over the irregularities of the surface to which it 

 was attached. As growth proceeded the shell not only 

 extended beyond the area of attachment, but also increased 



