STATION IN FRESH-WATER MUSSELS. 281 



These tables do not need any further comment : barring certain 

 local irregularities, which can be accounted for by scarcity of ma- 

 terial, the law is clearly shown, that in the upstream direction the 

 diameter decreases, and that it decreases very gradually. The con- 

 ditions in the Holston, and chiefly in the Clinch and Powell are, 

 indeed, classical. 



It should be added that I have seen a single individual from 

 Little Tennessee River, Coytee, Loudon Co., Tenn., which has the 

 diameter of 58 per cent. (= pilaris). Little Tennessee goes into 

 the Tennessee below Knoxville, and thus this agrees well with the 

 figure for specimens from Knoxville (57 per cent.). 



I further collected a large number of sipecimens in French Broad 

 River, at Boyd Creek, Sevier Co., Tenn. This river unites with 

 the Holston to form the Tennessee just above Knoxville. I have 

 measured 16 specimens ; the max. is 64 per cent. ; the min. 47 per 

 cent. ; the av. 56 per cent. Also these figures are exactly what we 

 should expect. 



There is no question that all these shells are one and the same 

 species, for which the name pilaris, as the oldest, should be used, 

 and which changes in obesity from the large rivers towards the 

 headwaters. Since Lea distinguished, at the same time, two forms 

 which dififer chiefly in being more or less swollen, the two names 

 given by him {pilaris and lesueiiriana) should be preserved in a 

 varietal sense, and a third variety of great compression should be 

 added, described much later by Wright as bursa-pastoris. 



As pilaris corresponds to sitbrotunda, bursa-pastoris corresponds 

 to kirtlandiana of the upper Ohio drainage. In fact, it is so close to 

 it that Pilsbry and Rhoads^" have identified specimens from Wa- 

 tauga River as kirtlandiana. I have serious doubts that the two 

 forms can be kept apart, when the locality is unknown. In my 

 large material I can see only one difference, that is, that bnrsa- 

 pastoris is a smaller shell than kirtlandiana, and reaches a greater 

 degree of compression. This opens a very pertinent question as to 

 nomenclature, since also pilaris (and lesueiiriana) apparently repre- 

 sent only a small race of sitbrotunda. I shall not go any further 



12 Pr. Ac. Nat. Sci., Philad., 48, 1896, p. 502. 



