174 Annals of the Carnegie Museum. 



5. These dimensions correlate with one another throughout the 

 environment. 



Anticipating to a certain extent results obtained in another investi- 

 gation, it may be said, that if, measurements from shells collected 

 from all parts of the Ohio Valley (rather than those solely from the 

 headwaters) be compared with those from Lake Erie, the Lake Erie 

 shells are distinguished by their almost perfect correlation of dimen- 

 sions as above, as distinguished from the negative aspects evinced 

 by the polymorphic Upper Ohio forms, coming really, as I hope I 

 have shown, from a variety of environments rather than a single 

 fairly constant one. Uniformity in environment, therefore produces a 

 uniformity of effect upon these species of Naiades in most cases. 



X. Suggestions as to Causes of Facts. 



It is thought well to again give a short summary of the physical 

 and biological conditions present in Lake Erie and the Upper Ohio 

 Drainage as they appear to be concerned with the Naiades. 



Lake Erie. Upper Ohio Drainage. 



Water colder than in Upper Ohio, but Water warmer than Lalze Erie, greater 



witli more even regulation of temperature. extremes of temperature. Streams more 



Currents much less rapid than in streams; rapid than currents of Lake Erie and 



water less agitated, except by moderate more greatly agitated; frequent falls and 



currents as indicated; carrying but little rapids; short stretches of quiet pools, 



sediment. Bottom composed of pebbles. Rivers carry a load of debris which moves 



sand, or mixture of these, depending on quicldy over bottom, consisting of mud, 



region of lake, with coarser sediment de- glacial till, cobbles. Food conditions 



rived from wear of land. Temperature {due to extreme of temperature), are 



conditions favor a more uniform produc- less stable, even if at times more abundant. 

 tion of food, while the water contains 

 more lime. 



Remarks and Suggestions Concerning Results Obtained. 



(o) On Inflation of Shell. 



Mr. Calvin Goodrich in a letter to the writer (1916), remarks that 

 when colonies of shells are found in the lake, the members are some- 

 times rnoving about. Ordinarily, he states, it is not the usual thing 

 for Unios to. wander. This is indeed more frequently the case in 

 spring when changing conditions impel migration, or at times when 

 water becomes low in streams. We have seen that the Naiades of 

 Lake Erie as a rule have a higher degree of inflation than those of the 



