136 OYSTERS, AND ALL ABOUT THEM. 



colours of the shell are all determined by the edges of this 

 mantle ; and the whole of these characters differ so greatly 

 in the different species, and the result is so beautiful in 

 many, that a collection of shells is very interesting. 



The nucleus, or starting point from which the forma- 

 tion of the shell proceeds, is called the umbo ; and the 

 manner in which the additions are made is very various. 

 Sometimes the mantle edge secretes a great deal of matter 

 at one time of the year, and is nearly inactive, or only 

 pours out a thin secretion, at another ; and this will produce 

 a shell with ridges and furrows parallel to the edges of the 

 shell, which are called sulci. 



If the mantle secretes at certain points, in larger 

 quantity, and but little between these points, or if it be 

 folded or puckered, and the folds remain so during the 

 whole of the growth, then ridges and channels are formed, 

 stretching continuously from umbo to margin. If the 

 margin of the mantle is much folded and thrust out during 

 secretion, it sometimes results in long points or projections, 

 which reach far beyond the rest of the outside of the shell. 



In the same way it will be seen that the lining and 

 colouring of the shell into patterns may be effected by the 

 partial and intermittent secretion of colouring matter. 

 The shell, while it is being extended, is also thickened by 

 a thin secretion poured out all over the external surface of 

 the mantle, and therefore all over the internal surface of 

 the shell. This latter secretion is always smooth and colour- 



j 



less, or with only a faint unvariegated pink or purple tint. 



The two valves of the shell are united 



by the mantle, and at or near the umbo of each valve there 

 is a hinge surface upon which the valves open. This hinge 

 has often a complex system of teeth, which, while they 



