272 



most variable of shells, and the exact limits of the species and its 

 various forms are not yet well worked out. The lake examples, thoug-h 

 differing considerably from those of the neighboring rivers and from 

 river shells in general, do not exhibit a very large range of variation. 

 They are all markedly dwarfed, the average length being about 2 1-2 

 inches or 63.5 mm. All have a well-developed rounded posterior ridge. 

 The epidermis is deeply stained, that of the exposed portion of the 

 shell being a rich yellowish brown, while the anterior portion, in the 

 living shell buried in the soil of the bottom, is a deep shining brown 

 black. The anterior margin is not nearly so heavy and produced as one 

 frequently finds it in river examples. The beaks of the lake shells are 

 not so angular as they usually are in river shells, and the high wavy 

 ridges are more numerous and pronounced. In the Maxinkuckee shells, 

 also, a number of fine hair-like lines or ridges, much like growth lines, 

 extend along the posterior border of the umbone, parallel with the 

 posterior ridge of the earlier stages of the shell. 



The nacre of the lake shells is a rich rosy salmon. Unlike the salmon 

 color of "Aiwdonta saliiionea", this is a natural color, not due to diseased 

 conditions; the nacre surface is very smooth and the color extends deeply 

 into the shell. In some cases the inner nacreous surface appears to be a 

 secondary thickening of the shell, laid on the older portions like an 

 enamel. Below this extra nacreous deposit the growth lines are very 

 distinct on the inner surface of the shell. The rest periods are distinct 

 black lines, often plainly visible through the translucent shell when 

 held up to the light. Rays are always invisible by reflected light in 

 the lake shells, but in some examples they wei-e visible by transmitted 

 light. The animal has orange-colored flesh. The few living examples 

 examined indicate that parasites are common; one contained three old 

 Atax ypsilophorns, and several young. 



One gravid example was found, October 17, 1907. The youngest 

 example found was 42 mm. long and exhibited four rest periods. 



7. Lam]}silis gluns (Lea). 



Fairly common in the main lake; dead shells are often found along 

 shore, and occasionally the living mussels are to be seen in shallow water 

 at the various mussel beds at the lake. It is quite abundant along the 

 edges of the thoroughfare joining the lakes, and is common in Lost Lake. 



