68 PALAEONTOLOGY 



in thickness by the addition of internal layers, which 

 gradually smoothed over the irregularities of the inner 

 surfaces but left those of the outer surfaces unchanged. 



Internally, there are no hinge-teeth ; there is a large 

 triangular elastic ligament, extending from the umbo to 

 the hinge-line. When the valves are closed, the outer 

 part of the ligament is probably under tension and the 

 inner part under compression. The central part is 

 thicker than the anterior and posterior, so that in the left 

 valve the ligament-area appears divided into three parts, 

 the central concave, the others flat : the whole ligament 

 area is marked by fine horizontal striations. 



There is only one adductor muscle-impression, posterior 

 in position, the anterior adductor being entirely aborted 

 in the adult: the shell is therefore mono-myarian. The 

 interior is sub-nacreous, the iridescence being faint. 



The external ribs give the valve-margin of the left 

 valve, which projects beyond that of the right, a coarse 

 crenulation. As this is traced towards the hinge it 

 becomes supplanted by a slightly deeper-set series of 

 crenulations, counterparts to which appear in the right 

 valve, together forming an approach to a taxodont denti- 

 tion. These "teeth" do not interlock tightly, however ; 

 they are only found in a few species of Ostrea ; they are 

 certainly not the relics of the hinge-teeth of the ancestors 

 of the Ostreidtz. 



The oysters (including allied genera as well as Ostrea 

 itself) have the strongest shells of any lamellibranchs, in 

 the sense of standing wear and tear b3St : they may be 

 washed out of their original deposit, rolled about by 



