TESTS OF THE CONCHULINA. O 



and N. bigibbosa, which seldom depart from the type, 

 whilst the large protrusions forming horns or exten- 

 sions externally on the tests of such species as Difflugia 

 oblong a, D. acwminatii, and Nebela bipes, often show 

 great variability in form and size ; such variation is 

 frequently found in a single locality, but is usually 

 greater when collections from different localities are 

 compared, and is more marked still when the speci- 

 mens come from different continents. 



Varietal and specific names have frequently been 

 assigned to apparently distinct varieties which sub- 

 sequent investigation has shown to be connected with 

 each other by an unbroken series of slight modifica- 

 tions; these names, although perhaps not strictly 

 justifiable, may still in many cases be useful or even 

 necessary to an accurate tabulation of local variations. 



A consideration of the methods by which Rhizopod 

 tests are constructed may afford a clue to an explana- 

 tion of the difference in the potentiality of variation 

 in these different structures, since we find that those, 

 portions or elements which are secreted involuntarily 

 are more stable in form and position than those 

 appendages which are either placed in position or 

 built up by the animal voluntarily, as we may perhaps 

 say, by means of its pseudopodia ; but further observa- 

 tions of tests in process of construction are much to be 

 desired. 



A feature which is usually reliable for specific dis- 

 crimination is the form of the aperture of the test and 

 the nature of its border or margin. Usually these are 

 verp constant for each species ; among the Euglyphx, 

 for instance, it is possible to identify nearly every 

 species if only a single aperture scale is available ; on 

 the other hand, although the Arcellse all have circular 

 apertures, the same species in perhaps the same locality 

 will provide tests with singly- and doubly-invaginated 

 apertures, which may be either plainly circular or 

 crenated ; this variability in the Arcellse is not common 

 in Europe, but is frequently met with in Xorth America. 



