(b GENUS XIV. OSTREA. 



creases with age, and in some species extends to a conside- 

 rable length, as in O. spondyloides, comucopicr, &c. ; most of 

 these can open their valves but a very little way, especially 

 when they get old. The animal protrudes neither tube nor 

 foot beyond the margin of the shell. Some species in this 

 division have equal valves, as O. Malleus, Vulsella, &c. : these 

 have not only a hollow for the cartilage in each valve, but 

 also another very close to it (/. 39) ; they are said to fix them- 

 selves by a byssus which passes through this second hollow. 

 There are also some other shells placed by Linnaeus in this 

 division, as 0. Perna, Isogonum, &c., which have the hinge 

 very different from all the rest ; there are no teeth at the hinge_, 

 but it consists of an uncertain nvimber of grooves or hollows 

 for holding the cartilage, ranged side by side in a straight line 

 (f. 40) : in some species there are but three or four of these 

 grooves, while in some others, as 0. Isogoyiimi, there are often 

 as many as fourteen or fifteen ; perhaps the number may in- 

 crease with age. The grooves are exactly opposite in each 

 valve, and appear each to hold a separate cartilage ; both the 

 valves are very little convex ; the animals are said to fix them- 

 selves by a byssus. 



Total, 31 101 4 136 



