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 O. subrotunda, as- 
sociated with Jens of moderate obesity (max. 62 per cent., min. 55 
percents). 
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 subrotunda. 
Loc. No. Max. Min. Av. 
Paint Rock River. 
IPR IRGC oe oe bidmevenceoad bares 14 50 46 54 lens 
MER CTICOMMm Petters. mo eroneto teeters 12 53 46 49.6 lens 
EfrolllivaelN Ce ereaacra tock entices I 48 48 48 lens 
PrIMCeLOM be tehuaaaeyto cetera 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 offer, at the present time, 
for the phenomenon that a species changes its obesity along the 
course of a river or rivers. 1 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 1s 
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 torulosa (Raf.), turning, in the upper 
Ohio into Tr. rangiana (Lea), and in the upper Tennessee into Ty. 
gubernaculum (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 which positively do 
not follow it, and exhibit the same obesity all along the course of a 
river in which they are found. 
