276 



out of a possible ele\-en the curve representing shells from the lower bay falls between 

 the other two. 



The most salient feature of this graph is the closeness with which it agrees with 

 the graph representing the number of ridges per millimeter of the shells from the 

 same three localities (fig. 106). While the relation is indeed an inverse one, it in- 

 dicates none the less the consistent action of en\'ironment in producing variation. 

 In other words, we have studied these groups of shells from two entirely different 

 points of approach, and have arri\ed in each case at the same conclusion, namely, 

 that the three different types of environment have produced three rather distinct 

 tvpes of variation in the shell of Teredo, and that in each case the intermediate environ- 

 ment (the lower bay) has produced a type intermediate between those of the middle 

 and the upper bay, representing the extremes of en\ironment. 



y. 



Mean Mode 

 Goat IB. - 47 50 

 Diunbarton- 48 50 

 Crookett - 55 55 



-1 L 



J L 



--, \ 



Index of auricle. 

 Fig. 111. Graph representing index of auricle of shell of Teredo at three different stations. 



V.\RiATiON IN Surface Sculpture 



Shells from the upper bay are generally more irregularly sculptured and rugose 

 than those from other portions of the bay. They are less transparent and lustrous; 

 the periostracum is thicker and more opaque, and considerably roughened across the 

 middle median area. The lines of growth are more prominent, indicating greater 

 lapses in the progress of growth than occur under the more constant conditions of the 

 middle bay. 



It has already been shown that the denticulated ridges are less numerous and 

 less closely spaced on shells from the upper bay than on those from the middle and lower 

 bay. A microscopic study shows that the differences are carried even to the details 

 of the denticles on the ridges. On the shells from the upper bay, with their fewer 

 and more widely spaced ridges, these denticles are more individuated and sharply 

 outlined, having a distinctly serrate appearance (fig. 112, 4). Shells from the middle 

 bay, on the other hand, have more close-set and less sharply delineated denticles, 

 which under the low power of the microscope, present a beadlike appearance, very 

 different from that just described (fig. 112, 2). Here once more we find that shells 

 from the lower bay occupy a position intermediate between the other two types 

 (fig. 112, J), with a tendency to resemble more nearly the form from Goat Island. 



Variation in Color 

 The shells from Crockett (see fig. 109) 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, 



