l8o BULLETIN OF THE BUREAU OF FISHERIES. 



is based upon the examination of the shell margin in mussels collected at various seasons 

 of the year and of mussels which had been placed in wire inclosures on the bottom of 

 the stream after having been accurately measured. The results from these plantings 

 were fragmentary because of the accidental destruction of most of the inclosures. In 

 one case, however, he found specimens which "when placed in the inclosure in August, 

 1909, and measuring 18 mm. in length, had reached, at the time of their examination in 

 June, 1910, a length of 26 mm." He reports that other similar investigations are in 

 progress, the results of which we shall await with interest. 



Since no accurate observations on the rate of growth of fresh-water mussels have 

 ever been made, we have attempted to secure definite data bearing upon this problem. 

 The data obtained are derived from two entirely different lines of observation, as indi- 

 cated by the headings of the sections which follow, and although meager they show 

 that with better facilities it should not be difficult to follow individual mussels from the 

 juvenile to the adult stages, and thus to determine their rate of growth in an accurate 

 manner. 



GROWTH OF MUSSELS IN WIRE CAGES. 



While engaged in mussel investigations at La Crosse, Wis., during the summer of 

 1908, we collected a number of young clams (fig. 68, pi. xvii) belonging to 16 different 

 species, and after weighing and measuring them accurately they were distributed in wire 

 cages, which were then anchored by long wires in midstream to the piers of a bridge over 

 the west channel of the Mississippi River opposite La Crosse. One hundred and sixty- 

 three small mussels, belonging to the following genera and representing both thin and 

 thick shelled forms, were planted out in this manner: Alasmidonta, Anodonta, Lampsilis, 

 Obliquaria, Obovaria, Plagiola, Quadnda, and Unio. 



Some of the cages contained only a single specimen of each species represented in it, 

 in which case an absolute identification would be possible, should the cage be recovered 

 later, while, if two or more individuals of a species were put in a cage together, only 

 specimens of practically the same size were selected. In the latter case it would of 

 course be impossible to subsequently distinguish an individual mussel, and only the 

 average rate of growth could be determined for the individuals present. It was assumed 

 that mussels of the same size and under the same conditions would grow at practically 

 the same rate. 



These plantings were made at inter\'als from June 29 to August 10, 1908. An 

 opportunity did not present itself to make an attempt to recover the cages for over two 

 years, but in November, 1910, Dr. R. E. Coker, who knew of the experiment, made a 

 search while on a visit to La Crosse and was fortunate enough to find 2 of the 1 1 cages 

 planted by us in 1908. One of the cages was deeply buried in the mud and all of the 

 mussels in it were dead; as they showed little or no growth, they were evidently killed 

 shortly after the planting. In the other cage, however, 6 living mussels were found, 

 as follows: 3 Lampsilis ventricosa, i Obovaria ellipsis, 1 Quadnda solida, 1 Anodonta 

 imbecillis. These 6 mussels, with the exception of the specimen of Obovaria ellipsis, 

 were readily referred to definite individuals as recorded at the time the cage was set out. 

 The comparative measurements and weights are given below. 



