STATION IN FRESH-WATER MUSSELS. 275 



average, are more swollen than old ones. This does not, however, 

 introduce a serious error, since from most localities young and old 

 shells are at hand. On the other hand, this serves to explain, in 

 part, the irregularities observed. Very old shells are often un- 

 usually produced or elongated at the posterior end, and this tends to 

 lower the percentage of the diameter. Just such shells have been 

 found frequently in the Ohio near Pittsburgh, and these have, con- 

 sequently, unduely depressed the average. Also the young shells, 

 or, for that matter, shells of about the same size, when compared 

 among themselves, show the decrease of the obesity in the upstream 

 direction. The same phenomenon is found in other forms to be 

 discussed later. 



Fusocnaia subrotunda also enters several tributaries of the 

 Ohio. First of all, we have it in Elk River in West Virginia 

 (tributary to Kanawha). The Kanawha empties into the Ohio in 

 a region where typical subrotunda is found. I have no material 

 from this river. But in Elk River is a form, which I have called 

 F. subrotunda leiicogona, and which I have characterized as a 

 *' rather small and somewhat flattened subrotunda," also differing 

 by (mostly) pale soft parts and eggs. I doubt now the propriety 

 of introducing a separate name for this form, for it might very 

 well be included under both, subrotunda and kirtlandiana, these also 

 having sometimes pale soft parts. The following table gives the 

 measurements of the Elk River specimen. 



Loc. No. Max. Min. Av. 



Shelton 12 52 45 49.5 kirtl. (subrot.') 



Gassaway 12 53 38 48 kirtl. (subrot.) 



Sutton 12 49 43 46 kirtl. 



The decrease of the average diameter is also here clearly shown, 

 the maximum being found in the two lower stations (with specimens 

 which would fall under subrotunda). The minimum has been ob- 

 served in the middle station, but only in one individual, and another 

 with the diameter of 41 per cent., while all the rest do not fall 

 under 45 per cent. This is one of the irregularities which should 

 be expected. 



Another river which contains this species is Beaver River in 



