282 



of the biochemical or other aspect of the organization of the animal which tends to 

 asymmetry. 



We have found in our study of pallets that the range of this type of \ariation is 

 so great as to render it extremely difficult to recognize variations resulting from 

 ecological factors. It has been noted, however, that pallets from the upper bay tend 

 to have a heavier periostracum than those from the middle and lower bay, which 

 corresponds with our observation concerning the periostracum of the shell (p. 278), 

 which was found to be thicker and rougher in the case of specimens from the upper 

 bay. It may be stated also, as a fact of general observation, that the chitinous tips 

 of the pallets tend to be darker in color in specimens from the upper bay than in those 

 taken elsewhere. This is not always true in indi\-idual cases, as will be seen b>' refer- 

 ence to figure 114; the tips of pallets 3b. 3d. and 3g. in the Goat Island series are 

 almost as dark as the darkest of the Crockett pallets ( Ig) ; but the proportion of pallets 

 with a notably dark distal portion is greater among the pallets from the upper bay. 



It should be mentioned in this connection, however, that pallets taken near the 

 mud line of a pile, or from waters containing much decaying organic matter, are 

 likely to be much stained, as was observed in the shells (p. 278). It is necessary, 

 therefore, to be cautious in interpreting such data. 



Systematic Be.\ring of Vari.\tions 



The data presented in the foregoing pages appear to demonstrate that certain 

 variations occurring in the shell of Teredo navalis in San Francisco Bay have a definite 

 relation to corresponding ecological conditions. This ine\-itably suggests some dis- 

 cussion of the systematic treatment to be accorded such en\-ironmental forms. 



The type of variate occurring in the upper bay has been described by Bartsch 

 (1921) as Teredo heachi, n. sp., San Pablo Bay being designated as the type locality. 

 The diagnostic characters which distinguish this form from the type of Teredo navalis 

 are not definitely stated. But we note in the description mention of a "strong posterior 

 auricle . . . umbones and a streak in the middle median portion . . . rose colored . . . 

 dental ridges . . . very strongly denticulated . . . strong auricle . . . marked by rough 

 lines of growth . . . center of the median portion is marked by a roughened area . . . 

 auricle is marked by strong, curved lines of growth . . ." etc. The emphasis on the 

 large auricle, color, strong denticulation, median roughened area, and prominent lines 

 of growth, as well as the type locality given, identify this teredo as the environmental 

 form which we have considered typical of the upper bay. The wide variation we have 

 described in these characters obviously precludes such ^•ariants from specific rank. 

 The question remains as to whether heachi should be retained as a sub-species, or 

 included in the sjnonymj' of navalis. In case of the former alternative, a subspecies 

 would have likewise to be established for the form from the lower bay and perhaps, 

 also, as further study might show, for each of several intermediate environmental 

 forms at other localities, the Goat Island form being considered typical. It might 

 even be necessary in some cases to establish subspecies for teredos from the top and 

 the bottom of the same pile. 



It will be recalled that the localities from which shells were taken for this study 

 were chosen as representing the extremes of environment in which Teredo occurs in 

 San Francisco Ba},-. Also, in attempting to establish the effect of environmental 

 conditions in producing variation, we have naturally and legitimately emphasized 

 the differences rather than the similarities observed among specimens from the locali- 

 ties in question. We would call attention to the fact that, while the extreme differences 



