5>n 



Till: AUl-l.'KAX XAirUAUST 



[Vol.XLIV 



size is eliminated, while the factor of form remains. 

 Tims a shell 20 mm. high with a greatest shell-aperture 

 of 12 mm. would fall in the 60 per cent, class (20: 12 = 

 60 per cent.), as would also a smaller shell 15 mm. high 

 with a greatest shell aperture of 9 mm. (15:9 = 60 per 

 cent.), while shells of the same height as the first, but with 

 a 14 mm. greatest shell aperture, would rightly represent 

 a variation in form since they fall into a different (70 

 per cent.) class (20:14 = 70 per cent.). This distinc- 

 tion mav be more apparent l>v reference to Fig. 2 where 



single representatives of all the different classes of var- 

 iants that appeared in a certain thousand-lot of shells 

 are arranged to show this point. Here the shells in any 

 horizontal row are the same height, and have, therefore, 

 presumably reached the same stage of growth, but at the 

 same time they are all unlike in form since those at the 

 left have larger "greatest shell apertures" than those at 

 the right. On the contrary, all the shells in any vertical 



