68 THE NAUTILUS. 



unlike in the shape of their valves as well as in 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 a septum. A shell having the same shape as the typical 

 Mytilus bifurcatus 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 bifurcatus in 

 company with young examples of Mytilus californianus Conr., and 

 some goose barnacles. One specimen was 1 of an inch from umbo 

 to ventral margin, and in its widest part I of an inch. It was 

 curved as in the type. There were three other shells, all like thi& 

 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- 

 cal Mytilus bifurcatus in appearance. 



My confidence in the constancy of the form of Mytilus bifurcattis 

 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 M. 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 is a 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 



