132 MEMOIRS OF THE CAENEGIE MUSEUM. 



I 



in exchange) I feel quite sure, that Grier is right in thinking that this shell repre- 

 sents a very peculiar phase of L. costnta. The whole outer aspect of this shell is 

 quite unlike that of the normal form, but, of course, the sculpture of the posterior 

 slope and the hinge do not leave any doubt as to its systematic relationship. As 

 the above measurements show, there is some variation in shape, and furthermore 

 the color of the epidermis may be lighter or darker, sometimes inchning more to 

 brown, sometimes to rusty. Greenish tints are rarely seen. 



Localities represented in. the Carnegie Museum: 



Lake Erie, Presque Isle Bay, Erie, Erie Co., Pennsylvania. 



Lake Erie, La Plaisance Bay, Monroe Co., Michigan (C. Goodrich). 



Distribution and Ecology (See fig. 12). Type locality: Lake Erie; Presque 

 Isle Bay, Erie, Erie Co., Peimsylvania. Type set: Carnegie Museum Cat. No. 

 61.4223 (not 61.4720, as Grier states!). 



I know this form only from the Lake Erie shores in Pennsylvania and Michi- 

 gan, but according to Sterki it also occurs on the Ohio shores, and very likely will 

 be found elsewhere in Lake Erie. Walker's (1913, p. 22) S. costata from Lake Erie 

 is undoubtedly this form. The specimens collected by myself are all from the 

 north shore of the bay "Big Bend," where I found them alive in one to three feet 

 of water, on sand and fine gravel, without vegetation, and at places, which are 

 exposed at times to a considerable surf. This form is not abundant there. Some 

 specimens were found among a scanty growth of rushes {Juncus americanus). 



Note: Since it is possible that L. costata of western New York may have come 

 by way of Lake Erie, it is interesting to observe that the typical L. costata is found 

 in this region, as indicated by De Kay's figure (1843, PL 14, fig. 226) from Oswego 

 River (Lake Ontario-drainage). This possibly is a case parallel to others men- 

 tioned above, where the lake-form, when migrating into the tributaries, assumes 

 again the shape of the normal creek-form. The matter, however, is not perfectly 

 clear, for there is the possibility that western and central New York was reached 

 by way of the upper Allegheny. 



Subgenus Pterosyna Rafinesque (1831).**^ 



Pterosygna Simpson, 1914, p. 490; Ortmann, 1914, p. 43. 



Type Unio complanatus Barnes. 



Only one species is known to belong to this subgenus. 



*^ Pterosyna, and not Pkrosijgnaj as Simpson writes. 



