STATION IN FRESH-WATER MUSSELS. 307 



smaller streams, and I have only a few specimens from the lower 

 Clinch and the lower Holston. These shells are 0. subrottinda, as- 

 sociated with lens of moderate obesity (max. 62 per cent., min. 55 

 per cent.). 



In a tributary of the Tennessee in Alabama, the law again be- 

 comes visible. Material from Paint Rock River has been examined : 

 although only the form lens is represented, it shows distinctly a 

 decrease of the obesity in the upstream direction, and at the lower- 

 most station specimens are found which are very close to subrottinda. 



Loc. No. Max. Min. Av. 

 Paint Rock River. 



Paint Rock 14 59 46 54 lens 



Trenton 12 53 46 49.6 lens 



Holly Tree i 48 48 48 lens 



Princeton 12 52 43 49.3 lens 



The irregularity at Holly Tree, of course, is due to scarcity of 

 material. 



These are the instances I am able to ofifer, at the present time, 

 for the phenomenon that a species changes its obesity along the 

 course of a river or rivers. I am quite sure that, in the upper Ohio 

 and upper Tennessee drainages, there are hardly any other in- 

 stances of this kind, for I am very familiar with these faunas. 

 There is only one additional species both in the Ohio and Tennessee 

 drainages, Lampsilis ovata (Say), which develops a variety, L. ovata 

 ventricosa (Barnes), which prevails towards the headwaters, and is 

 exclusively found in certain smaller creeks. These forms, however, 

 do not so much differ in obesity, but in other characters (develop- 

 ment of posterior ridge and truncation of posterior slope). 



Also possibly some species of Truncilla might show the same 

 changes in obesity (Truncilla tontlosa (Raf.), turning, in the upper 

 Ohio into Tr. rangiana (Lea), and in the upper Tennessee into Tr. 

 gubernaculnm (Reeve)). But these require further study. 



I have no doubt that in other river-systems other species or 

 groups may be found which are governed by the same law\ But, 

 on the other hand, there are surely other forms zvhich positively do 

 not folloiv it, and exhibit the same obesity all along the course of a 

 river in which they are found. 



