TESTS OF THE CONCHULINA. 5 
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 
oblonga, D. acuminata, and Nebelu 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 
very constant for each species; among the Huglyphe, 
for instance, it is possible to identify nearly every 
species if only a single aperture scale is available; on 
the other hand, although the Arcellz 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 Arcelle is not common 
in Europe, but is frequently met with in North America. 
