3^ut-s hells and Mussels 57 



sions of the ends of tliese muscles; these are the 

 points to wliich they were attached. The hinge- teeth 

 on the valves of the Nut-shells are simple in character, 

 but we shall find in other species that they gradually 

 became more complex. 



There are three other native species of Nut-shells 

 whose peculiarities may be briefly noted in contrast 

 with those of the common species. The Furrowed 

 Nut-shell {N. sulcata) has a larger, more triangular 

 shell, with five ridges parallel with the lower margin ; 

 dull olive. Found locally in from 3 to 50 fathoms 

 where the bottom is of sandy mud or clay. The 

 Shining Nut-shell (lY. nitida) is more acutely tri- 

 angular, yellowish brown, and very glossy; mantle 

 fringed. Sand and gravel from extreme 

 low water to 86 fathoms, but not very 

 common. The Thin Nut-shell (i\^. tenuis), 

 though still triangular, has a tendency 

 to roundness of outline, but flattened 

 from side to side; brownish yellow, thin, and glossy. 

 Found locally, in sand and mud from 25 to 100 

 fathoms, chiefly along the Scottish coasts, the north- 

 east of England, and east and south of Ireland. The 

 external colours given above are those of the living 

 epidermis or periostraciimi, which invests and protects 

 the shell from the action of gases dissolved in the 

 water. Beneath this covering the actual surface of 

 the shell may be entirely different in hue, but where 

 the epidermis is permanent it will be more convenient 

 to indicate its colour rather than that of the shell. 



Closely allied to the Nut-shells are the four native 

 species of Leda, but they are readily distinguished 

 by several differences in form and structure. One 



