REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. I 79 



coming in. This kind of an explanation has been offered, by the students of animal 

 behavior in recent years, to account for the formation of aggregates in a great variety of 

 the lower organisms; and it appears the most reasonable one in such cases as the one in 

 hand, where there is no evidence that the gregariousness is due to a definite recognition 

 of the presence of other individuals. 



RATE OF GROWTH. 



It has been quite generally believed, by those investigators who have given their 

 attention to this matter, that the mussel shell grows during the warmer months of the 

 year and that in winter there is no appreciable addition to its margin. When growth 

 begins again in the spring, the winter's rest has left a mark which appears as a dark 

 line on light-colored shells or as a deeper groove in others where the color is not so con- 

 spicuous. Finer lines may be found between these rings of growth, but the latter, like 

 the rings of a tree, mark the years. It is certain that these more conspicuous lines or 

 "rings," as we may term them, indicate an alternation of growing and resting periods in 

 the formation of the shell. It is not entirely certain that a single growth period must 

 always correspond to a single j-ear; for, when any lot of shells is carefully examined, 

 some will be found in which the "rings" are distinct and strongly suggestive of an annual 

 increment, while others of the same size may not show these rings in any such distinct 

 fashion, and one is forced to conclude either that the annual rings, if such they be, are 

 not always clearly to be seen or that some mussels may grow at a very different rate 

 from others. The examination of any considerable number of shells leads to the belief 

 that even if the annual-ring theory can be proved conclusively the rings are often not 

 sufficiently distinct from the intervening lines to give an unquestionable record of the 

 age. 



Assuming that these rings, when clearly seen, do represent years, it would seem that 

 the shell grows very rapidly during the first few years of the mussel's life and after that 

 much more slowly. To judge from the lines alone, we should say that many of the large 

 Quadrula shells had reached one-half their size in ten or a dozen years and then taken 

 forty or fifty for the remainder, so closely set are their later rings of growth; and that 

 shells of these species can not reach the most desirable commercial size in a less period 

 than twenty or thirty years. Since these are regarded as the best of all button shells, 

 the outlook may seem discouraging, because, like hardwood timber, the best shells take 

 too long to grow. 



The "ring theory" if proved would not, however, make the situation so discourag- 

 ing as might seem from the species of Quadrula; for we have in some members of the 

 genus Lampsilis shells which are almost if not equally desirable, and such evidence 

 as we have from the rings indicates that shells like these may reach a commercial size 

 in a very few years and that even forms like the quadrulas may become marketable 

 within a period of four or five years. 



In a recent paper, Israel (191 1) has reported his conclusion that there is no winter- 

 rest period and that more than one ring may be formed in a single year. This statement 



