MUSSELS AND PECTENS 35 



eric name signifies that it is a partition-bearer, and it 

 was given on account of the fact that a little shelly 

 deck is stretched across a small part of the interior 

 of each valve, near the umbo. The shell itself is 

 strong, has a thick, dark epidermis, and is very 

 pearly within, where the color varies from white to 

 dark purple. Outside, the ribs increase in number 

 towards the edge of the shell, sometimes seeming to 

 actually fork into two. Sometimes the outside is 

 exceedingly eroded, and almost all signs of ribs dis- 

 appear, or the surface may become incrusted with 

 foreign substances. The Septifer spins a strong 

 byssus and attaches itself to stones, where it may 

 dwell in safety. Occasionally a specimen is found 

 which is nearly twice as long as the picture. On the 

 whole, this is a very interesting shell, and it should 

 be sought for, especially in the south. 



Closely allied to the common mussels is another 

 group of shells which for some reason are popularly 

 known as "Horse-mussels." The true name is Modio- 

 lus, 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 cover- 

 ing, filled with bits of broken shells. Some of them 

 live in the mud, with only a small part of the shell 

 above the surface. In all of these shells the umbo 

 is not 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 



