BIVALVES. 11 
margin (fig. 1); and the pointed, the posterior. 
The prominent part of each valve near the hinge 
is the wmbo, u, which, when it isin the middle, the 
shell is said to be equilateral ; but in the painters’ 
mussel, we observe that the portion of the shell 
lying in the front of the umbo is shorter than that 
behind it, and the shell in this case is said to be 
mequilateral. Behind the umbo is a ridge, com- 
posed of a horny elastic substance, called the 
ligament, 1, which is a mechanical contrivance by 
which the valves are opened. The depressed 
space in front of the umbo is the lunule. The 
length of the shell is measured from the anterior 
to the posterior side ; its breadth 1s the perpen- 
dicular distance from the umbo to the front; its 
thickness is the diameter through the closed 
valves. Hxternally the valves are marked by 
concentric lines of growth, which diverge from 
the umbo, which is the point from which the 
growth of the valve commences. 
The shell is composed of layers of animal matter 
impregnated with carbonate of lime, and con- 
sists of three structures :—an outer horny layer, 
called the epidermis, which does not contain calca- 
reous salts; and which may be removed bysteeping 
the shell in an acid solution, when the epidermis 
alone remains. Under the microscope, it exhibits 
a cellular structure in some parts, and a granular 
in others. Beneath the horny layer is a stratum 
