146 THE AMERICAN MIDLAND NATURALIST 
* With regard to the exceptions in the general trend of the correlat- 
ions as presented for certain species, the cases of Pleurobema and 
Lasmigona are difficult to comment upon on account of the small 
amount of material. Nor am I able to add anything for the other 
species furnishings exceptions to the general trend of correlations 
beyond suggestions presented in another paper, (7, p. 174), beyond 
stating the results may have been occasioned by unusual variants. 
The fact that juvenile shells exhibit mostly the same correlations 
as adult shells may indicate that the lines along which they develop 
are largely predetermined. Predetermination in this case, however 
might indicate a sufficiently rigid uniformity of environmental con- 
ditions to guide the shells in their development along the lines of their 
ancestors, or the inherent nature of the mussel protoplasm itself. 
IX.—PossisLte Sources or Error. 
It is true that in the experiments dealing with mussel culture I 
have cited, that all the shells were subjected to identical environment- 
al conditions, which in itself probably insures a greater uniformity of 
results for the types of environment studied. A small amount of the 
material needed to obtain as complete a series as possible came from 
other localities in L. Erie besides Presque Isle, the source of most 
of it. There need be raised on this account no serious objection 
as from the aspect of environmental effects upon shells, the broad 
idea of the lake environment being more important in the present 
discussion than that of any separate effects of the smaller localities 
involved in it. If anything, it is more probable that greater balance 
has been given the calculations on account of some of the ma- 
terial being from different localities. 
Undoubtedly the results would be more satisfactory to some, if 
it had been possible to compare equal numbers of shells in all cases, 
but the results obtained have so fitted in with what was previously 
known of the growth process in these animals, that it is felt no serious 
objection can thereby be interposed. 
X.—BIBLioGRAPHY. 
1. Ball, G. H. “Variation in Fresh Water Mussels’ Ecology, 
Vol. III, p. 93-121. : 
2. =<Coker, R. E., ‘Shira; A. F.; Clark, Hi Ws, aloward Ac) 
“Natural History and Propagation of Fresh Water Mussels’. Bull. 
U. S. Bureau of Fisheries, Vol. 37, 1919-20, p. 125-129. 
