REPORT OF THE DIRECTOR 1920-21 1/9 



about twenty-five specimens. The largest found measures 42 mm x 

 27.5 mm. One fragmentary specimen, if complete, might be a little 

 larger than this specimen, but there is not enough difference to count 

 in the general run. Other specimens measured range from 33 mm x 

 20.3 mm to 41 mm x 34.8 mm; and the fragmentary specimens 

 seem to run about the same size. I should say that in general the 

 Valcour island material runs about the same as the South Hero 

 material, with perhaps a smaller representation of the larger-sized 

 specimens. From the above measurements it is seen that the 

 Valcour specimens run one-half, and less, the size of the recent 

 specimens; in the South Hero material, the largest specimen is 

 about three-fifths the size of the largest recent specimen, and a large 

 part of the average material is about one-half the size of recent 

 specimens. At Cumberland Head the relations stand much the 

 same, though the larger specimens here are more abundant. 



There is one other species, Cylichna alba, which occurs in 

 a dwarfed form in the Champlain valley. This species has been 

 found only at Port Kent, and even there is relatively infrequent. 

 About 115 specimens were found during the course of several days' 

 collecting, all running much smaller than the Canadian specimens. 

 The largest Port Kent shells are about one-half the size of those 

 from Canada, the majority, however, are much smaller, ranging 

 from two-fifths down to one-third the size of Canadian forms. (See 

 plate 2, figures 8, 9.) 



Just as it is found in the Baltic, so here, together with the dwarfing 

 of species goes a decreasing thickness of shell. The little 

 Cylichna alba, just discussed, has a very thin shell, so thin 

 that even in working the specimens out of the sand with the point 

 of a small knife-blade many were broken. Of the other shells, 

 Yoldia arctic a, Macoma groenlandica and M y a 

 arenaria perhaps show the most noticeable changes. Yoldia 

 a r c t i c a, through the Champlain area, from Port Kent southward, 

 shows the same characteristics as Cylichna alba. The shells 

 are of a paperlike thickness and very easily broken in collecting. 

 For this reason most of those collected at Chimney Point, Vt., the 

 most southern locality, are in a fragmentary condition. Even the 

 smaller specimens of Macoma groenlandica from the 

 Montreal area are stoutly built and not easily crushed. Though 

 there are slight variations in some localities, in general there is 

 a gradual decrease in thickness of the shells of this species going 

 southward, until at Crown Point even the largest shells are very 

 easily crushed into numerous pieces between the fingers. At Cumber- 

 land Head and South Hero the shells of Mya arenaria 



