26 ILUNOIS BIOLOGICAL MONOGRAPHS [124 



of Naiades to varieties on the basis of compression in the headwaters of 

 streams. The same species varies in obesity, but it is the same species, 

 whether thin or fat. Other characters are usually present which separate 

 the allied species. 



Table VI. Species or Uniontdae Found Above and Below Hoicsr Park Dam 



Both Above and Below Dam Below Dam Only 



paroa anodontoides 



ferussacianus dlipsiformis 



grandis ligamentina 



imbecillis mtdtiradiata 



edentulus lackrymosa 



pavonius melanevra 



luteola xvardii 



lienosa R. tuberculata 



complanata T. tuberculata 



rubiginosa circulus 



undulata clava 



pustulosa glans 

 costata 

 marginala 

 cocfineum 

 ventricosa 

 compressa 



In the Big Vermilicwi this variation in compression is marked in several 

 species. Thus Rotundaria tuberculata, Pleurobema coccineum, and Amblema 

 are more compressed than are individuals from the Wabash River below 

 the junction of the Big Vermilion with that river. Quadrula pustulosa is 

 smaller than the same species lower down in the Salt Fork, and the same 

 may be said of Alasmidonta marginata and Strophitus edentulus. That the 

 rule does not always hold good is shown by the variation of Fusconia rubi- 

 ginosa which is abundant in most parts of the Big Vermilion and its tribu- 

 taries. Measurements are given in Table VII, showing the length and 

 breadth of several species in different parts of the Salt Fork from below 

 Urbana to the Big Vermilion. The percentage of width to length is also 

 shown. 



It will be seen that the average index for the first lot is 42 per cent and 

 for the last lot, 46 miles down stream, is almost the same, 45 per cent. 

 These averages compare well with some of those given by Ortmann (1920: 

 283). It was observed, however, that in the Salt Fork and Big Vermilion 

 the obese individuals occurred with the compressed specimens the former 

 increasing in ratios as the distance down stream increased. The variety 

 wardii of Quadrula metanevra occurred in two places in Salt Fork but always 

 in company with the typical form. In the cases cited above ecological 

 features cannot be called into account in locating the cause of the com- 



