176 Annals of the Carnegie Museum. 



firml}- packed gravel. I\Iany of its favorable conditions are found in 

 Lake Erie as may be observed. Possibly its most enjoyable condition 

 is found in a lucustrine en^•ironment, as indicated by the tabulation, 

 which shows that of seven dimensions taken, five are greater there. 

 Proptera is a lively shell, crawls about much, likes a steady current 

 with rocks, gravel, and sand firmly packed. It would seem a greater 

 height is more useful to it in moving about the rivers with their con- 

 ditions than in Lake Erie. Here paucity of material compelled me to 

 use that of the Ohio and I am uncertain as to the conclusions to be 

 drawn, at least with regard to their stability. L. ovata ventricosa likes 

 quiet pools and eddies, gravel partly covered with mud. Here again 

 difficulty with material arises. L. ovata passes upstream into L. 

 ovata ventricosa. To what extent this took place in my material, 

 and what precisely the difTerences were, there was no way of telling, 

 due to the puzzling intergrades. Mr. Goodrich states that in Lake 

 Erie this mussel has a trick of burrowing completely below the surface 

 and passes its gills fan-like from the shell, in which case the height of 

 the shell might facilitate the burrowing process. However, other 

 species do the same. Wilson and Clark (54) remark that this mussel 

 becomes smaller towards the lakes, and perhaps we have here an- 

 other evidence of compensatory growth. Specimens of L. ovata from 

 "big rivers" are higher than those of the lakes. 



(c) On ths Comparative Size of the Anterior and Posterior Portions of 



the Shell. 



It will be remembered that shells from the Upper Ohio were found 

 to be better developed anteriorly and less posteriorly than those of 

 Lake Erie. The heed of sometimes having to move against opposite 

 and unfavorable influences is met in the Naiades by the development 

 of a foot, situated anteriorly. The physical nature of conditions in 

 Lake Erie offer less impediment to such movements. Thus it may 

 happen that the greater use of the foot in the Ohio would result in 

 its greater enlargement, and have an ultimate effect in the development 

 of the shell covering it. Such hypothesis could only be proven experi- 

 mentally, inasmuch as other factors may be concerned. Sell found 

 that the anterior part of the shell was best developed in rivers, and 

 from the results of another investigation I may state that in the 

 Monongahela, where conditions more closely resemble those of Lake 



