238 GENEEAL HISTOEY OF THE INFUSORIA. 



SO as to include not only the particular genera enumerated, but also the' 

 families Anuiehce and AreeUma of that naturalist. 



The term Monothalamia contrasts \Yell mth that of Poh/thalamia, expresses 

 the fact, and involves no hyi)othesis as do Ehrenberg's words Monosomatia 

 and Pol y so mat i a, which are foimded on his belief in the colony-like aggrega- 

 tion of several individuals ^vithin a Poraminiferous shell. 



The Monothalamia of Schultze (as before remarked) do not precisely cor- 

 respond with the one-celled group of either of the other authors named ; for, 

 besides the Monostec/ia of D'Orbigny, it comprehends the Arcellina of Ehren- 

 berg and a few other new genera. The groupings and relations of the several 

 species are represented in the appended table exhibiting Schultze's system. 



In the classification of the Monothalamia certain and constant characters are 

 deducible from the shells, whilst those ch'a^Mi from the soft parts, from the 

 length or tenuity or mode of termination of the pseudopodes, are of compara- 

 tively secondary importance, and not to be relied on alone. Definite charac- 

 ters are deiivable from the figm^e, size, composition, sculptimng or appendages, 

 and colour of the entii^e shell, from the presence of a single large aperture or 

 of many small pores, and from the form of the apertiu'e and of its margin ; 

 consequently it is in the shells of the Polythalamia that we must seek generic 

 and specific distinctions. As animals, they have all alike the same sarcode sub- 

 stance, which extrudes similar variable fibres : hence any diversities observed 

 in its colour or transparency, in its contents, or in the maimer in which the 

 processes are extruded or other^^ise comport themselves, serve but a sub- 

 ordinate purpose in the scheme of classification. On the contrary, the cha- 

 racters of the shells are, Avithin certain limits, determinate and fixed. They 

 are derivable from the figure, size, coloiu% and consistence of the shell ; from 

 the markings, processes, pores, and slits occupying its surface; from the 

 relative position and figiu'e of the several chambers; from the mode and 

 degree of their connexion ; and from the presence or absence of large apertm^es 

 in company Avith the usual foramina ; and last, not least, from the intimate 

 structiu'e of the shell. Dujardin recognized the value of the shells to supply 

 the basis of a classification of the Rhizopoda ; but he had recoui'se to the form 

 of the variable expansions to make his primary di\-ision, " although," as he 

 remarks, *' it has no absolute value." He arranged all the Rhizopoda, with 

 the exception of the Amcehce (which he treats as a distinct family), into two 

 sections, — one having a single unilocular shell mth a single large apertui-e ; 

 the other a foraminiferous compound shell, or one having several aggregated 

 chambers, each ^^ith a simple orifice, as represented by the tribe Miliola. It 

 is in the subdivision of these sections that he employs characters derived from 

 the variable processes. Thus he separates the first into — 1. those animals pro- 

 vided with short and thick processes rounded at the extremities, viz. Difflugia 

 and ArceIJa ; and 2. into those having filiform expansions, acutely drawm out 

 at the ends. The latter di\ision is more largely represented ; and he separates 

 its numerous species into three tribes, viz. Trinema, with a lateral orifice ; 

 Euglypha, with a tuberculated or areolated shell and few sim2:)le exj^ansions ; 

 and Gromia, with a membranous spheroidal shell and expansions, thick at 

 the base, but very long and branching. He has not attempted the classifica- 

 tion of the whole of the Foraminifera, but restricted his accoimt to some few 

 genera which he has foimd in a living condition. 



D'Orbigny instituted five orders of the PolijtliaJamia, viz. — 1. Stichostegia, 

 having the ceUs arranged one above another in a straight or shghtly-curved 

 line ; 2. Helicostegia, ^ith cells disposed spirally around an axis ; 3. Ento- 

 mostegia, having the chambers alternating and coiled spii\ally ; 4. EnalJo- 

 stegia, with alternating but not spirally-disposed chambers ; 5. Agathistegia, 



