102 MEMOIRS OF THE CARNEGIE MUSEUM. 



this, the rays are better visible, and the growth-rests become more marked. When 

 older the epidermis becomes darker, and may be even blackish, but in many old 

 specimens it remains light, chiefly so at or near the posterior end. This part of 

 the shell is often covered with a growth of other organisms. The regularity of the 

 growth-rests is rather variable, but in well-developed specimens it is quite striking. 

 In shape young specimens are more like normal dilatatus. In fact sometimes they 

 are indistinguishable except by the color. 



Localities represented in the Carnegie Museum: 



Lake Erie, Presque Isle Bay, Erie, Erie Co., Pennsylvania (type locality). 

 Lake Erie, off North shore of Presque Isle, Erie, Erie Co., Pennsylvania (depth 10 feet). 

 Lake Erie, Cedar Point, Erie Co., Ohio (C. Brookover); La Plaisance Bay, Monroe Co., Michigan (C. 

 Goodrich); Port Colbome, Welland Co., Ontario, Canada (C. Goodrich). 



Distribution and Ecology (See fig. 10 and Walker, 1913, fig. 2, p. 30) : Type 

 locality: Lake Erie, Presque Isle Bay, Erie, Erie Co., Pennsylvania. Type set: 

 Carnegie Museum Cat. No. 61.4628 (not 4268, as Grier states!). 



This form is exclusively known from Lake Erie. According to my observa- 

 tions it lives in Presque Isle Bay, preferably along the North shore of the bay, in 

 the characteristic fine sand, in from one to five feet of water, and chiefly at the 

 edge or within the Jmicus americanus formation (among "rushes"). It also is 

 frequently found among Chara patches. However, I also found it on the south 

 shore of the bay in gravel and shingle, and one specimen was obtained by the 

 "sand-sucker" in the open lake, in about ten feet of water. 



Sterki (1907a, p. 392) and Walker (1913, p. 22) are the only authors, who 

 previously have mentioned this form from Lake Erie, but without separating it 

 from the main form. Grier (1918) first recognized it as a variety. 



Note: It is interesting to compare specimens obtained under similar ecological 

 conditions at other localities. A very remarkable form is E. dilatatus from Chaut- 

 auqua Lake, Chautauqua Co., New York. I have eighteen specimens collected 

 by various parties at Bemus Point and Celeron. This is a form distinctly inclining 

 towards the var. sterkii. It is rather small (longest 79 mm.), is also slightly more 

 swollen than the true dilatatus, and has the beaks a little more anterior; but with 

 regard to color, the Chautauqua form does not differ from dilatatus. This is, of 

 course, no intergrade (genetically), but has developed independently, coming up 

 into Lake Chautauqua from, the upper Allegheny. 



In Conneaut Lake, Crawford Co., Pennsylvania, a form of E. dilatatus exists, 

 which is not at all different from the normal one, except that it is rather small. 

 It is rare in this lake, but more abundant in the outlet, and assumes, in French 

 Creek, to which the outlet flows, the typical size. 



