20 MUSSELS OF CUMBERLAND RIVER AND TRIBUTARIES. 



RELATIVE ABUNDANCE OF DIFFERENT SPECIES. 



In forming an estimate of the relative abundance of the different 

 mussels in the various beds many things have to be taken into con- 

 sideration. 



For the clammer's purpose, a count of his entire catch would give 

 the most reliable data, but this is usually impossible. It is almost as 

 satisfactory to take the successive hauls as they come and count the 

 various species in each; the greater the number of hauls counted the 

 more accurate the results obtained. 



From the viewpoint of the conchologist, however, such an estimate 

 is in reality only a measure of the extent to which the species in ques- 

 tion is capturable by the clammer's gear, and for the following 

 reasons : 



There are a number of species which never ''bite" the hooks on a 

 crowfoot dredge, or which do so very rarely. Such species may be 

 plentiful in a mussel bed and yet never appear in the clammer's 

 hauls. 



Again, some mussels are found only in small numbers and around 

 the edges of a bed. The clammer makes his hauls where the shells 

 are most crowded, through the center of the bed, and may miss these 

 altogether. 



The clammer throws away the mussels that are too small to use as 

 well as those whose shells are too thin or too highly colored. Such 

 shells ought to enter into the percentages as much as the more valu- 

 able species, but they do not appear in the clammer's hauls. 



Different methods of clamming produce very different results in 

 the proportion of shells obtained. The crowfoot dredge, the rake, 

 the tongs, and wading each secure an unduly large number of some 

 species and an unduly small number of other species. 



To enumerate all the shells obtained by all the methods would give 

 the most accurate results, but that is obviously impracticable. When 

 the water is low the clammer gets quite a different proportion of 

 species, and may even get different kinds of mussels from those 

 obtained when the water is high. 



Each of these considerations has been kept in mind while making 

 out the percentages; the clammer's hauls were counted; all the piles 

 of culls were carefully examined; all the specimens possible were 

 secured by wading along the edges of the beds; account was taken of 

 the various shells found in muskrat piles; the relative stage of water 

 was noted, and, so far as could be done, allowance was made for it. 

 Then, too, there has been a careful consideration of numerous cir- 

 cumstances which can not be shown to the reader, but which result 

 from the authors' experiences at the different stations. Notwith- 

 standing all these efforts, the numbers must still be regarded as 



