76 GENUS XIV. OSTREA. 



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

 rable length, as in O. spondyloides, cormicopice, &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 Linneeus in this 

 division, as O. 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 number of grooves or hollows 

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

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

 grooves, while in some others, as O. Isogommi, 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 theiiir 

 selves by a byssus. 



Linnaeus has described Gmelin has added Dr. Turton Total. 



In the 1st and 2d 7 

 divisions, j 



In the 3d division, 3 4 



In the 4th division, 9 25 



Total, 31 101 



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