34 WEST AMERICAN SHELLS 
Closely allied to the common mussels is another 
group of shells which for some reason have been 
popularly known as ‘‘horse-mussels.’’ The true 
name is Modiolus, formerly spelled Modiola, 
which means a little measure, or drinking cup. 
They are usually more solitary in their habits 
than the true mussels, and they often spin a kind 
of covering, filled with bits of broken shells. Some 
live in the mud, with only a small part of the shell 
above the surface. In all of these shells the umbo 
is not at the extreme point, as in the mussels, but 
a part of the shell projects by the hinge, forming 
a rounded lobe. 
Of the species found on this coast there is first 
Modiolus modiolus, Linn., the Great Horse-mussel, 
most abundant in northern waters. In Puget Sound 
there have been found specimens nine inches long 
and four inches in diameter, standing perpendic- 
ularly in the mud. The epidermis is strongly 
bearded near the edge of the shell, while the buried 
parts are strongly eroded. They grow rapidly. 
Modiolus capax, Conr., is a similar species from 
the south, though I have grave doubts whether it is 
really distinct from the last species. I have gath- 
ered them abundantly about San Pedro. When 
the epidermis peels off from the outside of the 
shell the latter often appears of a brick red color. 
Modiolus réctus, Conr., the Straight Horse- 
mussel is shown in Figure 16. The shell is long 
and narrow, thin and delicate. The epidermis 
near the hinge end is dark brown and glossy; in 
front it is light brown, with numerous chaffy 
hairs; internally the shell is white. The common 
