OF THE PKOTOZOA. RHIZOPODA. 221 



progressive formation of the shell. To recall the particular points of interest, 

 in the primary stage the Difflugia was seen covered by an integument, but 

 having processes extruded from various parts of its surface, so far resembling 

 the Corycia of Dujardin, — whilst in a later stage all processes were withdi-awn, 

 except Ithose at the one end where the single large orifice or mouth is placed, 

 and thus came to resemble the Pami:>liagus of Bailey. 



Calcareous-shelled Monothalamia are represented by the genera Squamu- 

 lina, Orhulina, and Cornusjmri. Such shells are brittle, and in all essential 

 featiu^es resemble those of the next-considered family, the Fo7rim{nifera. 



The shells of Monothalamia are generally coloui'ed. When seen, as they 

 often may be, empty, they have an orange-yellow, a bro^^m, or brownish-black 

 tint. This coloiu' is acquii^ed by age ; the younger the being the less is it, 

 ccetens parihus, colonized. In the youngest, as before noticed, the whole sub- 

 stance and its commencing envelope are quite colourless. Most shells are 

 also translucent or. diaphanous when empty ; but in others the colour is so 

 deep, that, when filled, scarcely anything of the contained substance is dis- 

 cernible through them. The testae of Difflugice are mostly opaque. The sur- 

 face of the shells is subject to numerous modifications. Occasionally it is 

 uniformly smooth ; but many, which so seem when occupied by the animal, are 

 found when empty to be really finely sculptured (XXI. 11-15). 



Arcella liyalina is represented by Ehrenberg to have a smooth and coloui^- 

 less testa ; A. vulgaris and A. dentata, one superficially divided into facettes ; 

 A. aculeata, A. spinosa, and A. caudicola, a delicately hispid shell. Where 

 the intersecting lines or ridges are not sufficiently developed to produce fa- 

 cettes, they give rise to areolae and an areolated or reticulated surface. The 

 surface is beset with rounded tubercles or eminences in Euglypha tuherculata, 

 and by spirally-disposed polygonal depressions (alveola) in Euglypha alveo- 

 lata (XXI. 11). In Difflugia acanthophora (Ehr.) (XII. 64), the surface 

 looks as if covered by scales laid on in an imbricated manner and in. a spiral 

 direction. The same species and Euglypha alveolata (XXI. 11) afford instances 

 of testae armed with large and strong spines. This same Difflugia presents 

 likewise an example of the mouth of the shell being strongly serrated. Several 

 Arcellina have small depressions or pits on their sui'face, which at fii^st sight 

 resemble pores, e. g. Arcella Ohenii ; and both this species and A. vulgaris, 

 according to Perty, present very numerous striae diverging from the centre of 

 the closed end, and concentric ciixles, the outermost of which in Arcella Okenii 

 are dentated, and follow the stellate expansions of the shell (XXI. 15). 



Among Difflugice the shell is more often rough from the adhesion of parti- 

 cles of sand and of other extraneous substances (e. g. in D. iwoteiformis, J). 

 gigantea, D. acuminata), but in others consists of a smooth membrane, as in 

 D. Enchelys, D. ohlonga, and D. glohidosa. Moreover, Ehrenberg enumerated 

 D. ciliata, D. acanthophora, and other species as having an areolated surface, 

 D. ampulla as punctated, D. dryas and D. reticulata as cellular, D. Bructerii 

 as rugose, and D. striolata as striated. He further states that D. ciliata has 

 a bristle or cii'nis in the centre of each posterior areola. 



Where spines or other elevations of the smface — or, in fact, markings in 

 general, exist — they may not be imiformly disposed, but be produced in larger 

 number or of larger dimensions in some parts than in others. Thus Ekrenberg 

 signalizes an irregular disposition of the spines in Arcella aculeata ; and not 

 uncommonly such processes are produced only from the vicinity of the mouth. 



These examples will sufficiently illustrate the diversity of sm^face jDreva- 

 lent among monolocular shells ; but these shells moreover differ as remark- 

 ably among themselves in size, figure, and character of the margin, and 

 likewise in the relative position of their mouth, foramen, or orifice. These 



