62 OF THE FORAMINIFERA GENERALLY. 



Class RHIZOPODA, Order Reticularia. 



Sub-order, imperforata. 



Test membranous ..... Famil}' Gromida. 



Shell porcellanous „ MUiolida. 



Shell arenaceous ..... „ Lituolida. 



outline of the system in which the Foraminifera first liad their true place assigned to them, the classi- 

 fication of M. Dujardin will always have an historical value, although its incompleteness has been 

 made apparent by subsequent researches. The Amihiens constitute the second, the Rhizopodes the 

 third, and the Actinophryens the fourth family of his Infusoria ; but as he distinctly states (p. 240) that 

 the structure of the animals is essentially the same in the first two cases, it is rather to be wondered 

 at that he should have limited the name Rhizopods to such as have the body enclosed in a testaceous 

 envelope. This envelope, he says, varies in consistence from a simple flexible membrane to a thick 

 calcareous shell, either solid or porous. But he does not regard these differences as equal in 

 importance to those presented by the form of the pseudopodial extensions of the sarcode-body, 

 according to which the group may be divided into two sections ; of which the first (corresponding to 

 Ehrenberg's family Arcellina) includes only the Arcella and Diffiugiw, whose pseudopodia are short, 

 thick, and rounded at their extremities, whilst the second comprehends all those whose pseudopodia 

 are filiform and much attenuated towards their extremities. This second section he subdivides into 

 three tribes ; the first composed of the genera Trinema, Euglypha, and Gromia (all discovered by him- 

 self), which are distinguished from Diffiugia only by the attenuation of their pseudopodia; the 

 second is composed of the single genus Miliola, which agrees with the ordinary Foraminifera in the 

 possession of a calcareous shell, whilst in having but a single large aperture from which the pseudopodia 

 extend themselves it corresponds with Gromia ; and the third includes the Foramimfera proper, 

 which he supposed to be all furnished with porous shells for the passage of pseudopodia from the 

 general surface of the body. It is remarkable how little change is required (and this rather in the 

 application of terms than in re-arrangement) to bring this outline into conformity with the more com- 

 plete system which subsequent research enables us now to frame. For if we extend the application 

 of M. Dujardin's term Rhizopodes not only to the Amibiens which precede them but to the Actino- 

 phryens which follow them in his classification, and transfer to the former from the central group the 

 genera Arcella and Difflugia whose animals are of the Amoeban type, and to the latter the genera 

 Trinema and Euglypha which are Actinophryan, we find the central group thus restricted to correspond 

 exactly with our Order IIeticulakia ; and the limitation or non-limitation of the pseudopodia to the 

 single or multiple aperture would probably have been adopted by i\I. Dujardin as the basis of his primary 

 divisions of that group, if he had been aware that Miliola, so far from being exceptional among Foraminifera 

 in this respect, is in reality the type of an extensive series. — Whilst this sheet is passing through the 

 press, I find that Prof Reuss has recently propounded to the Imperial Academy at Vienna (xci a) 

 a scheme of classification of which the principles are almost identical with my own. He considers the 

 composition and intimate structure of the shell to be characters of primary importance, and attaches but 

 little value in comparison to plan of growth. He still retains the distinction into Monoihalam.ia and 

 Polythalamia (which he terms Monomera and Polymera), but expresses himself doubtfully as to its 

 value. 



