STATION IN FRESH-WATER MUSSELS. 283 



sents, in the more obese form, distinctly the shell called by Lea 

 trigomts, and thus I shall use this name, and restrict myself to the 

 two forms flava and trigona, and their interrelation, leaving the 

 decision as to the standing of undata to further investigation. Ac- 

 cording to Utterback,^^ the latter also passes into trigona. 



Loc. No. Max. Min. Av. 



Ohio River. 



Coraopolis 14 63 49 54 flav. (trig.) 



Monongahela River. 



Elizabeth i 49 49 49 flav. 



Charleroi 8 66 50 56 trig, {flav.) 



Millsboro i 47 47 47 flav. 



The following are tributaries going into the Monongahela at and 

 above iNIillsboro. 



Loc. No. Max. Min. Av. 



N. Fork Ten Mile Creek, Amity.. 2 48 48 48 flav. 



S. Fk. Ten Mile Cr., Waynesburg 6 48 4J 44 flav. 



Dunkard Creek, Wiley 2 47 43 45 flav. 



Dunkard Creek, Mt. Morris 10 50 41 47 flav. 



The law is again apparent here : the most compressed forms are 

 found in the tributaries, the most swollen ones in the larger river. 



Going up the Allegheny River, and ascending one of its tribu- 

 taries, Crooked Crock, the law is not so clearly shown for the reason 

 that this form is rare in the Allegheny proper, a;nd is entirely missing 

 in the headwaters. Yet indications are seen in Crooked Creek. 



Loc. No. Max. Min. Av. 



Allegheny River. 



Godfrey 3 44 41 43 flav. 



Johnetta 2 55 50 53 flav. {trig.) 



Kelly 2 42 42 42 flav. 



Crooked Creek. 



Rosston 29 51 38 43 flav. 



South Bend 2 41 41 41 flav. 



Creekside 8 44 38 41 flav. 



That the compressed form is the one found in smaller streams, 

 is also shown by several isolated sets from other tributaries of the 

 Ohio, which have been measured, and the following are of interest. 



^^ Amer. Midland Naturalist (Notre Dame, Ind.), 4, '16, p. 21. 



