l8o NEW YORK STATE MUSEUM 



particularly were found in quantity and suggested a mass of broken 

 eggshells, a resemblance which is further carried out by their extreme 

 thinness and brittle character. The thinness of the shells of this 

 species at these localities is in striking contrast to the specimens 

 from Montreal and recent specimens. The Montreal specimens are 

 somewhat heavier even than the recent individuals and are three or 

 four times as heavy as the specimens from the Champlain area, 

 measuring even up to 2.6 mm, 3.5 mm, or even 4 mm, in thickness 

 at the thickest part of the shell. Saxicava rugosa shows 

 extremely heavy shells, even among the smaller sizes, from the 

 Montreal area (see plate 1, figures 4, 5). Some of them are much 

 heavier than the recent forms from Barden bay, Greenland, but 

 in general this species seems to have been less affected than the 

 others as regards thickness of shell. Saxicava rugosa near 

 Pebble Beach, south shore of Valcour island, is so abundant that 

 shells can be collected by the hundreds in a very short time. Here 

 the shells tend to run rather heavy, the largest ones approaching 

 the Montreal specimens, which in their heaviest expression have 

 a thickness of 2 mm to 2.5 mm and almost 3 mm in the thickest part 

 of the shell. At Port Kent and southward the shells run thinner 

 again, having their thinnest expression in the few specimens found 

 a few miles north of Port Henry. Here the thickness of the largest 

 specimens is no more than .5 mm. There is not sufficient data to 

 make similar comparisons for the specimens ofMytilus edulis 

 found in the various localities. 



Walther (1920, p. 210) points out that brackish-water conditions 

 are indicated also by insignificant constancy of form. This is shown 

 to some degree by Macoma groenlandica and Saxi- 

 cava rugosa, where the shells may be longer and narrower, or 

 shorter and wider than normal or show gradations between these 

 two forms. In Yoldia arctica from the Champlain area the 

 modified form of the shell is very noticeable, as shown by figure 7, 

 plate 2. In the recent forms and those from the Montreal and 

 Ottawa areas, there is a pronounced posterior extension or wing, 

 with subacute tip. The specimens from the Champlain area, Port 

 Kent and southward, possess this posterior wing, but it is shorter 

 and blunter, giving a squarish appearance to the posterior end of 

 the shell; in a large proportion of the shells, the wing is so blunted 

 at the tip that it is hardly recognizable as such. There are all 

 gradations between these two types of forms, and except for these 

 gradations the extreme forms of the Champlain area are so different 

 from the typical form from the vicinity of Montreal and Ottawa 

 that one would be inclined to regard them as belonging to another 

 species. 



