(8) 



guishable from the thick, rough "posterior" edge, which may 

 bear from one to eleven spines, each tipped hy a grain of • 

 quartz which looks like the cork in a bottle. In all but 

 the oldest shells, the outline of the mouth, which is sit- 

 uated at the top of a f-oj-inel-like infolding of the lower 

 surface, is plainly visible through the shell, even when 

 the organism is viewed fi^om the upper side. 



In order to permanently record the characters of an 

 individual, then, it was necessary only to transfer the 

 living organism to a mdnute drop of water on a slide and 

 to trace tfcie outline of sliell and spines with a camera 

 lucida, under the high power of the compound microscope. 

 By focussing down a little the outline of the mouth was 

 then added, "uch camera outlines v/ere made of every in- 

 dividual used in the statistical [iart of the work, and in 

 one of the selection experimemts . Some of them are repro* 

 duced, reduced in size, as illustrations for the present 

 paper . 



The chitinous shell surrounds an ameboid protoplast. 

 At the time of coll division t'ne protoplast swells, prob- 

 ably by the absorption of water, ana projects out of the 

 mouth of the old shell. This projection assumes the size 

 and shape of a new shell, but one with its "antero-posterior' 

 axis in just the opposite direction to that of the old 

 shell. To the surface of this mold flow various sand- 

 grains and diatom tests which have been picked up by the 

 parent organism and stored within its body. Kow, over the 

 entire surface of the exposed protoplast, a chitinous sub- 



