STATION IN FRESH-WATER MUSSELS. 273 



Thus it appears that in the Ohio, between Portsmouth and Pitts- 

 burgh, there is a tendency to decrease the diameter in the upstream 

 direction. The most swollen specimens, with the highest diameter, 

 and the highest average of it, have been encountered at the lower- 

 most station (Portsmouth), and the most compressed specimens, 

 and the lowest average, at the two uppermost stations. This gen- 

 eral law holds good also, when we now go up the Allegheny River, 

 and enter French Creek. (In the Allegheny proper, above the 

 mouth of French Creek, this species has not been found.) 



Loc. No. Max. Min. Av. 



Allegheny River. 



Natrona i 45 45 45 kirtl. 



Aladdin 3 47 45 46 kirtl. 



Godfrey 26 66 43 51 suhrot. {kirtl.) 



Johnetta 2 50 46 48 kirtl. (subrot.) 



Kelly 113 54 38 47 kirtl. {subrot.) 



Templeton i 49 49 49 kirtl. 



French Creek. 



Utica 4 43 40 42 kirtl. 



Cochranton 8 49 ss 4^ kirtl. 



Meadville 2 46 45 45.5 kirtl. 



Cambridge Springs 2 44 43 43.5 kirtl. 



Here we notice irregularities, indeed, in so far as, for instance 

 at Godfrey, the percentage increases again, and remains only a little 

 below 50 per cent, in the Allegheny, but sinks rather abruptly to a 

 little above 40 per cent, in French Creek. The latter phenomenon 

 may be due to the sudden decrease in the size of the streams. It 

 should also be noticed that both the maxima and minima decrease in 

 the upstream direction, and it always should be emphasized that the 

 material from the various localities is not uniform as regards the 

 number of specimens measured: thus irregularities should be en- 

 countered, and, of course, mathematical exactness cannot be ex- 

 pected in a biological object. 



Taking all the shells together, beginning in the Ohio at Ports- 

 mouth, and going up the Allegheny to French Creek, we notice the 

 general decrease of the obesity: the most swollen specimen was 

 found at Portsmouth (6y per cent.) ; the most compressed one (33 

 per cent.) in French Creek. Specimens from the lower parts of the 



