312 University of California Publications in Zoology [Vol.22 



power of the microscope, present a beadlike appearance, very different 

 from that just described (pi. 17, fig. 2). Here once more we find that 

 shells from the lower bay occupy a position intermediate between 

 the other two types (pi. 17, fig. 3), with a tendency to resemble more 

 nearly the form from Goat Island. 



Variation in Color 



The shells from Crockett (see plate 16) are very distinctly more 

 pigmented than those of either of the other series. This is true of 

 shells from the upper bay generally ; we have found it to be the case 

 almost without exception in shells from Mare Island, Crockett, Port 

 Costa, and Martinez, as compared with shells from Goat Island, 

 Angel Island, Oakland Harbor, and the San Francisco water front. 



Coloration when present ordinarily manifests itself chiefly in a 

 fairly broad brownish stripe from dorsal to ventral across the middle 

 median portion. This stripe varies in color from a pale brownish 

 yellow through varying shades of brown, often warmly tinged with 

 red, to a heavy sepia ; the color nearly always tends to fade out 

 toward the dorsal and ventral margins of the shell. In addition to 

 this median stripe we often find, more especially in shells from the 

 upper bay, a roseate or occasionally brownish color suffused over the 

 denticulated area (anterior median) ; in the more heavily pigmented 

 shells a distinctly reddish or brownish patch occurs on the auricle. 



As regards the absolute differences of coloration in shells from 

 the upper and the middle bay, it is difficult to make any statement 

 more definite than that the former manifest color in maximal, the 

 latter in minimal, amount. Shells from the upper bay are character- 

 istically marked with reddish or brownish color, while those from the 

 middle bay are lighter and more translucent, and often entirely 

 white. The Dumbarton shells more nearly resemble, in this respect, 

 those from the middle bay. 



With our present limited knowledge of the physiology of Teredo, 

 the basis of these differences in color is difficult to determine. The 

 color lies entirely in the periostracum, which may be scraped off, 

 leaving the shell perfectly white. This would suggest that the thick- 

 ness of the periostracum is at least a factor in producing depth of 

 color. It has been stated above that the shells from the upper bay 

 exhibit a thicker and rougher epidermis; it is natural therefore that 

 these shells should have the deepest color, while those from the lower 



