68 THE NAUTILUS. 
unlike in the shape of their valves as well asin the presence or ab- 
sence of a septum. Yet some shells of the latter resemble the former 
so closely that it is sometimes necessary to open each shell in order 
to distinguish one from the other. The approximation appears too 
close for not only a generic, but a subfamily distinction to be main- 
tained between them. It appears to rest upon the presence or ab- 
sence of aseptum. A shell having the same shape as the typical 
Mytilus bifureatus has, upon examination, revealed the deck or sep- . 
tum. On December 1, 1888, on one of the wooden piles of the old 
wharf at Santa Monica, Cal., I found shells of Mytilus bifwreatus in 
company with young examples of Mytilus californianus Conr., and 
some goose barnacles. One specimen was { of an inch from umbo 
to ventral margin, and in its widest part 2 of an inch. It was 
curved as in the type. ‘There were three other shells, all like this 
one, only smaller. They were together and attached either by their 
own or the byssus of M. californianus. Three shells were opened 
and the absence of a septum noted. One specimen got broken and 
one was sent to another Los Angeles collector. In an exchange 
with Mr. W. J. Raymond, of Oakland, Cal., the one shell that had 
not been opened was sent to him, and I was surprised when he wrote 
that he had found a good-sized “deck in it!”” They were all typi- 
eal Mytilus bifurcatus in appearance. 
My confidence in the constancy of the form of Mytilus bifurcatus 
was further shaken by receiving what appeared to be four young 
shells of Septifer bifurcatus that Mr. Raymond had received from 
San Diego. One of these was without a deck, and Mr. Raymond 
called my attention to it as a proof that I. bifurcatus could resem- 
ble, in shape, a Septifer more closely than a Mytilus. Here we 
have an illustration that a shell found among young Septifers, and 
their counterpart externally, is a Mytilus bifurcatus, and one shell,. 
in form, that looks like a typical M. bifurcatus, proves to be a Sep- 
tifer. 
The San Diego examples from Mr. Raymond all have purple in- 
teriors, and the Santa Monica example has a white interior. But 
some shells, collected at one of the “ Points” in Los Angeles County 
and sent for identification by Mrs. E. A. Lawrence, are also white 
in their interiors. But there isa marked difference between the 
Santa Monica Mytilus and those from the “ Point” and San Diego 
in their outward appearance. 
In order to determine the genus to which each belongs, the value 
