Structural Variation among the Diffiugian Rhizopods. 289 
or surface-markings of the test cannot be regarded as denoting 
generic distinction, I am unable to perceive any valid ground 
for separating Cyphoderia (Schlumb.), Difflugia Enchelys (Khr.), 
and Lagynis* (Schultze) from the typical genus Huglypha. 
The case is different, however, when we find that the test of 
one section of a subfamily is invariably chitinoid, and of another 
as invariably siliceous. I accordingly deem it necessary to place 
Cadium (Bailey)—a form I have met with in abundance, and 
the true siliceous nature of whose test I can certify—ain a distinct 
genus, along with the marine forms to which reference has been 
made. 
Under this view of the general affinities of the testaceous 
freshwater Rhizopods, Difflugia Arcella may therefore be regarded 
as the connecting link between the Difflugide and Euglyphide ; 
whilst a very cursory examination of the forms of the latter sub- 
family, in which the general axis of the test and of its aperture 
are not coincident, will serve to show that this peculiarity is 
merely the counterpart of the obliquity that has already been 
shown to pervade the tests of the marsupiform series of the 
Diffugide, and, lastly, that the apical appendage which fre- 
quently makes its appearance in Huglypha margaritacea, and is 
earried to such an extent in one of the new oceanic forms— 
namely Cadium caudatum(Wall.)—as to constitute a tail-like organ 
several times the length of the body of the test, is in like manner 
merely the homologue of the apical appendage of Difflugia acu- 
minata and the horned varieties of the globular series. 
But to conclude. Assuming from the facts which have been 
advanced that the shape, material, size, and colour of the Dif- 
flugian tests furnish characters so singularly prone to accidental 
variation as to yield no trustworthy criterion of generic or even 
true specific distinctness, and recalling to mind once more that 
the animal is in every instance specifically the same, it appears 
to me impossible to arrive at any other conclusion than that the 
whole of the subspecies, as well as their intermediate varieties 
(widely though some of these appear to differ from others 
in external features), have not only been originally derived by 
direct descent from a single progenitor, but do still continue 
to be produced by direct descent from varieties which have 
become permanent, and may, one and all, still be produced from 
a common archetype under the varying conditions to which 
these lower forms of animal life are universally subject. 
* My knowledge of this Rhizopod is derived exclusively from the de- 
seription and figures published by its discoverer, Professor Schultze (Ueber 
den Organismus der Polythalamien, p. 56, taf. 7 & 8); and, allowing due 
weight to the difference of habitat, I can perceive no valid reasons for con- 
sidering as only apparent the resemblance to Euglypha curvata (Perty) to 
which allusion is made in the definition. 
