I7 2 NEW YORK STATE MUSEUM 



around 17 mm to 18 mm in length by 14 mm to 16.5 mm in width. 

 At Crown Point, the largest shell found had a length of 15 mm and 

 a width of 11.6 mm. The large shells are few and run from a little 

 under 13 mm to 14 mm in length by 10 mm to 11 mm in width. 

 Average specimens measure 10 mm to 12 mm in length by 7 mm 

 to 10 mm in width. These measurements show that at Montreal 

 the largest shells of M. groenlandica found are about the 

 size of the average-sized recent individuals; while the average- 

 sized Montreal specimens run much under this. The largest shells 

 found at Crown Point are less than one-half the size of the largest 

 recent ones and less than three-fifths the size of the largest shells from 

 Montreal. The average Crown Point shells are half, or less, the size 

 of average recent shells, and three-fifths, or slightly over, the size 

 of average Montreal specimens. The Montreal specimens may be 

 regarded as typically marine, and those from Crown Point as 

 dwarfed. Other localities show various gradations between these 

 two points, but there is a general decrease in size southward. (See 

 plate 1, figures 6, 7, 8.) 



M. groenlandica in the vicinity of Ottawa runs smaller, so 

 far as specimens collected show, than at Montreal. The largest 

 specimens found at Green's creek measured 19.2 mm to 20 mm in 

 length by 14.7 mm to 17.5 mm in width ; a number of shells measured 

 from 17.2 mm to 18.3 mm in length by 14.3 mm to 15.6 mm in width; 

 the majority of the shells from here are even smaller, the average 

 running 15.2 mm to 16.5 mm in length by 12 mm to 13.7 mm in width. 

 The average-sized shells here then are 2 mm to 3 mm smaller than 

 at Montreal. At Cumberland Head, near Plattsburg, N. Y., the 

 largest shells, very few in number, run from 17.5 mm to 21 mm in 

 length by 15.4 mm to 16.3 mm in width; medium-sized shells measure 

 15 mm to 16.7 mm in length by 12.4 mm to 14.3 mm in width; the 

 majority of the shells are smaller yet, ranging in size from 13.8 mm x 

 12 mm to 14.6 mm x 11.9 mm. At McBride bay, South Hero, Vt., 

 these shells are smaller than at places farther south, except Crown 

 Point and Chimney Point. This may be due to lack of extensive 

 collecting, or perhaps an environment unfavorable to a better develop- 

 ment of the shells. The largest shell found measures only 17.5 mm x 

 14 mm; medium-sized shells measure from 13 mm x 10.5 mm up to 

 14.3 mm x 12 mm; the majority of the shells have the following 

 measurements or are smaller: 11.5 mm x 10.2 mm up to 13 mm x 

 10.7 mm. Near Burlington, Vt., and vicinity very few of the large 

 shells were found and these measured from 18.7 mm x 16.5 mm to 

 22 mm x 17.7 mm. The medium-sized shells running from 17 mm x 



