128 



BULLETIN OF THE BUREAU OF FISHERIES. 



It must be borne in mind that the conditions of life for mussels in an inclosed crate, 

 and relatively closely crowded together, are probably not nearly so favorable for growth 

 for the majority of mussels as are those of the natural river bottom, where the mussel 

 has a fair chance to assume its desired position and secures the full benefit of the food- 

 laden current. Doubtless the maximum rate of growth shown in the crate is more 

 nearly normal than the average rate. Our impression is that thick-shelled mussels, such 

 as the niggerheads, pig-toes, and pimple-backs, after they are half grown, increase 

 in size ordinarily at the rate of a quarter of an inch a year or less. If this be true, it 

 would require four years or more for a niggerhead mussel, under ordinarily favorable 

 conditions, to increase from a length of 2 inches to a length of 3 inches. Assuming that 

 the rate of growth is more rapid in early life, it may be inferred that niggerheads or 

 pimple-backs 3 inches in length are 10 or 12 years of age. Additional experiments 

 conducted under proper conditions are clearly wanted. 



A marked contrast in rate of growth is thus afforded by the species of Quadrula (and 

 others having generally similar character of shell), on the one hand, and those of Lamp- 

 silis, on the other. This was strikingly shown, in connection with the last experiment 

 described, by two examples of the yellow sand-shell, Lampsilis anodontoides, which were 

 not put into the crate but which must have found their way in by chance when still 

 small enough to pass through the screen wire of >^-inch mesh. Although the crate was 

 out only a little over two years, these two sand-shells were respectively 3.30 and 4.12 

 inches in length. Being elongate in form, they may have entered the crate when a 

 little more tian an inch in length. 



Table 14 embodies the result of measurements of length and counts of rings on 

 yellow sand-sheUs, Lampsilis anodontoides, from the vSt. Francis River, at Madison, Ark. 



Table 14.- 



-Classification op 40 Yellow Sand-Shells prom St. Francis River, Ark., According 

 TO Length and Age. 



° Shell with stunted appearance. 



The observations indicate that mussels of this species in the St. Francis River attain 

 a length of 4 to ^yi inches in 4 years, that they may attain a length of 4 inches in 3 years, 

 and that 6 years or more are ordinarily required to attadn a length of 5 inches. 



In summary, the rate of increase in length of fresh-water mussels varies from i^ or 2 

 inches per year for paper-shells (as Lampsilis Icevissima) to % inch (a little more or a little 

 less) per year for the niggerhead and related species, while an intermediate rate of ^ or i 

 inch per year characterizes the muckets and pocketbooks, and a slightly more rapid rate 

 the sand-shells. In general the rate of growth is so directly proportioned (in inverse 



