114 



anterior adductor cicatrices distinct, pit-like anil deep ; posterior adductor cicatrices shal- 

 low, confluent, that of the retractor pedis mnsole impressed at tip of the laterals and 

 below; pallial cicatrix evident, regularly impressed and linear; dorsal cicatrices sev- 

 eral, crowded, in the deep cavity of the umbones or on the margin of the plate 

 joining the hinge teeth; cavity oi the umbones rather deep; nacre purple, with 

 anterior margin usually white, whole i)osterior region beautifully iridescent. 



Some interesting features connected with the comparative dimensions of the 



sexes may be shown from this table of measurements. If the two longest males 



.".7.56 37.10 



be selected the ratio of length to height is = l.fiO + and =: 1.66 



23.44 22.32 



37.56 



In 

 :^7.10 



these same shells the ratio of length to width is as follows: 



2.00 and 



18.50 17.24 



2.15. 



■•V comparison of the same dimensions for the two longest females develops 



34.40 30.28 



= 1.55 and = 1.50. Comparing the lengths 



22.10 20.10 



the following ratios: 



34.40 



30.28 



with the widths the ratio established is 



1.76 ^ and 



= 1.83. The 

 1!).51 l(i.5U 



ratios show that the females are much wider than the males, a relation probably 



due to the requirements of the ctenidi'i of the female shells when functioning as 



gestatory sacs. So marked, even to casual observation, are these relations that it 



is an easy matter to select the sexes in any considerable number of shells. 



The habits of XJnio (jlani< are somewhat different from those of Unio parms. 



It more commonly affects gravelly beds, in shallow running water. The writer 



has taken the cormivciilus form in great abundance in the typical locality, whence 



it was traced into nearly all the streams of north (leorgia and Alabama, in tlie 



Gulf drainage. The ci/lindrellus form is very abundant in the smaller streams of 



