1922] Miller: Variations in Teredo navalis in Sim Francisco Bay 'Ml 



the relation is indeed an inverse one, it indicates none the less the con- 

 sistent action of environment in producing variation. In other words, 

 we have studied these groups of shells from two entirely different 

 points of approach, and have arrived in each case at the same con- 

 elusion, namely, that the three different types of environment have 

 produced three rather distinct types of variation in the shell of 

 Teredo, and that in each case the intermediate environment (the 

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

 middle and the upper bay, representing the extremes of environment. 



Fig. F. Curves representing index of auricle of the shell of Teredo at three 

 different stations. 



Variation in Surface Sculpture 



Shells from the upper bay are generally more irregularly sculp- 

 tured 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 process 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 closely spaced on shells from the upper bay than on 

 those from the middle and the lower bay. A microscopic study shows 

 that these 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 (pi. 17, 

 fig. 4). Shells from the middle bay, on the other hand, have more 

 close-set and less sharply delineated denticles, which, under the low 



