132 



THE NAUTILUS. 



Butte des Morts. Common on rock and sand bottoms in water 

 one to ten feet in depth. 



As in Proptera and Lampsilis, the lake environment has pro- 

 duced a shell somewhat different from typical gracilis, which is 

 a river species. Compared with gracilis from the Fox River at 

 Omro, the lake shells are rounder, higher in proportion to their 

 length, the dorsal margin is strikingly alate in the male, form- 

 ing a distinct wing, the shell is more compressed, the color is 

 usually pale straw-yellow with few or no radiating lines, and 

 the rest periods are very distinctly marked. The largest speci- 

 men from Plummers Point, Lake Butte des Morts (the type) 

 measures as noted below, a small specimen of gracilis horn Omra 

 being also added for comparison : 



Length 94, height 61, width 30 mm. Butte des Morts. Type. 



Length 100, height 67, width 35 mm. Omro, Fox River. 



Gracilis from Illinois and other localities in the Hinkley col- 

 lection (about 15 lots) are without the marked wing so well de- 

 veloped in the males of the Winnebago Lake individuals and 

 are also much larger. Female shells of lacustris are smaller, 

 more ovate and narrower, but there is not as great difference in 

 the dorsal wing as in the male shells. They are uniformly 

 yellowish and have well marked rest periods. 



Ortmann (1920, p. 249) has noted a difference between the 

 Lake Erie form of gracilis and the Ohio River form, but does 

 not believe the difference of enough value to distinguish. The 

 material from Wisconsin is quite strikingly different and the 

 separation of the lake form seems warranted. This, as well as 

 the other lake forms, are true ecological varieties, produced by 

 the change in environment. 



Strophitus edentulus rhombicus (Anthony). Lakes Winnebago 

 and Butte des Morts, on a rocky or sand bottom, in one to five 

 feet of water. The edentulus of the lakes differ markedly from 

 those individuals living in rivers. They are much smaller, 

 more regularly elongated or subelliptical, and the umbones are 

 more depressed. A large individual from Lake Winnebago 

 (Long Point Island) measures, length 57, height 35, width 

 23 mm. This is a characteristic lake form and is probably the 

 same as that mentioned by Ortmann as living in Lake Erie 



