FORAMINIFERA AND BADIOLARIA. 85 



up the coarser quartz-grains, unites them together with a ferruginous 

 cement secreted from its own substance, and thus constructs a flask- 

 shaped 'test' having a short neck and a single large orifice. Another 

 picks up the -finer grains, and puts them together with the same cement 

 into perfectly spherical ' tests ' of the most extraordinary finish, perfo- 

 rated with numerous small pores, disposed at pretty regular intervals. 

 Another selects the minutest sand-grains and the terminal portions of 

 sponge-spicules, and works these up together apparently with no cement 

 at all, but by the mere Maying' of the spicules into perfect white 

 spheres, like homoeopathic globules, each having a single fissured orifice. 

 And another, which makes a straight many-chambered 'test,' the conical 

 mouth of each chamber projecting into the cavity of the next, while 

 forming the walls of its chambers of ordinary sand-grains rather loosely 

 held together, shapes the conical mouths of the successive chambers by 

 firmly cementing to each other the quartz-grains which border it. To 

 give these actions the vague designation 'instinctive,' does not in the 

 least help us to account for them; since what we want is, to discover the 

 mechanism by which they are worked out; and it is most difficult to con- 

 ceive how so artificial a selection can be made by creatures so simple. 



470. VITREA. Returning now to the Foraminifera which form true 

 shells by the calcification of the superficial layer of their sarcode-bodies, 

 we shall take a similar general survey of the vitreous series, whose shells 

 are perforated by multitudes of minute foramina (Fig. 314). Thus, Spi- 

 rillina has a minute, spirally convoluted, undivided tube, resembling that 

 of Cornuspira (Plate xv., fig. 1), but having its wall somewhat coarsely 

 perforated by numerous apertures for the admission of pseudopodia. 

 The ' monothalamous ' forms of this growth mostly belong to the Family 

 Lagenida ; which also contains a series of transition-forms leading up 

 gradationally to the ' polythalamous ' Nautiloid type. In Lagena (Plate 

 xv., fig. 9) the mouth is narrowed and prolonged into a tubular neck, 

 giving to the shell the form of a microscopic flask; this neck terminates 

 in an everted lip, which is marked with radiating furrows. A mouth of 

 this kind is a distinctive character of a large group of many-chambered 

 shells, of which each single chamber bears a more or less close resemblance 

 to the simple Lasfena, and of which, like it, the external surface generally 

 presents some kind of ornamentation, which may have the form either of 

 longitudinal ribs or of pointed tubercles. Thus the shell of Nodosaria 

 (fig. 10) is obviously made up of a succession of lageniform chambers, the 

 neck of each being received into the cavity of that which succeeds it; 

 whilst in Cristellaria (fig;. 11) we have a similar succession of chambers, 

 presenting the characteristic radiate aperture, and often longitudinally 

 ribbed, disposed in a nautiloid spiral. Between Nodosaria and Cristel- 

 laria, moreover, there is such a gradational series of connecting forms, as 

 shows that no essential difference exists between these two types, which 

 must be combined into one genus, Nbdosarina ; and it is a fact of no lit- 

 tle interest, that these varietal forms, of which many are to be met-with 

 on our own shores, but which are more abundant on those of the Medi- 

 terranean and especially of the Adriatic, can be traced backwards in 

 Geological time even as far as the New Red Sandstone period. In 

 another genus, Polymorphina, we find the shell to be made up of lageni- 

 form chambers arranged in a double series, alternating with each other on 

 the two sides of a rectilinear axis (fig. 13); here, again, the forms of the 

 individual chambers, and the mode in which they are set one upon 

 another, vary in such a manner as to give rise to very marked differences 



