No. 473] 



MUTATION IN MOLLUSC A 



329 



(1) to a fat, robust shell, with a comparatively short spire (10). 

 In some specimens the whorls are flat-sided (4), while in others 

 they are convex, especially the last, which is very convex (3, 10). 

 The columella plait also varies in size and elevation and the 

 sutures vary in the degree of impression (compare 3, 4). Several 

 of these mutations, if we can so designate these variations, have 



been doscril)e(l as species or varieties; thus 10 is Lea's nultalliana 

 and 1 is Say's elodes. It will be seen that no line can be drawa 

 between 1 and 10 in the presence of the intervening figures. Take 

 away these connecting links and a very distinct variety remains. 



In the ]\Iollusca the factor of geographic variation plays a very 

 important part. The shells from the locality illustrated in Fig. 1 

 are rather small, measuring 23 mm. in length. Fig. 2 illustrates 

 the same species from Sugar Island, near Alpena, Michigan (col- 



