Structural Variation among the Difflugian Rhizopods. 339 



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 (Ehr.), 

 and Lagynis* (Schultze) from the typical genus Euglypha. 



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 — in 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 Difflugida and Euglyphidce ; 

 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 

 Difflugida, and, lastly, that the apical appendage which fre- 

 quently makes its appearance in Euglypha margaritacea, and is 

 carried to such an extent in one of the new oceanic forms — 

 namely Cadium caudatum^W^W) — 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- 

 scription and figures published by its discoverer. Professor Schultze (Ueber 

 den Organismus der Polythalamien, p. 56, taf. 7 & 8) ; and, allowing due 

 weight to the ditterence 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. 



