304 ORTMANN— CORRELATION OF SHAPE AND 



while D. dronias caperatus is the compressed form, with the di- 

 ameter less than 50 per cent., and without hump. But we must bear 

 in mind that some moderately swollen individuals have no hump ; 

 and that, more frequently, flat specimens may have more or less 

 distinct traces of this hump. 



The original measurements taken from Lea, Conrad, and Simp- 

 son, give for the typical form a diameter ranging from 50 to 84 per 

 cent., while the diameter of caperatus ranges from 42 to 48 per cent. 



I am able to submit the following table. 



Loc. No. Max. Min. Av. 

 Tennessee River. 



Tuscumbia 4 74 59 66 drom. 



Konxville i 58 58 58 drom. 



Holston River. 



McMillan 2 56 53 54.5 drom. 



Mascot 10 61 43 52 drom. {cap.) 



Hodges 3 58 50 55 drom. 



Turley Mill 3 63 52 56 drom. 



Noeton i 48 48 48 cap. 



Holston Station 3 56 47 52 drom. {cap.) 



Farther up in the Holston, this species has not been found. It is 

 clear that the form caperatus is not well developed here, and this is 

 also shown by the fact that traces of the hump are found to the 

 uppermost station, although, on the other hand, already one of the 

 specimens at McMillan is without any hump. In the Clinch and 

 Powell, conditions are more favorable to the development of the 

 caperatus-type, as is seen in the next table. 



Loc. No. Max. Min. Av. 

 Clinch River. 



Solway 2 53 49 51 drom. {cap.) 



Edgemoor 16 58 41 49 cap. {drom.) 



Clinton 6 52 43 47 cap. {drom.) 



Offutt 5 51 40 46 cap. {drom.) 



Clinch River Station i 46 46 46 cap. 



Powell River. 



Combs I 46 46 46 cap. 



Arthur i 42 42 42 cap. 



Combs is about equivalent to Clinch River Station. Also here 

 the development of the hump is variable, but specimens without it 



