VITREOUS FORAMINIFEKA 819 



do on a most minute scale, the ' tests ' they construct, when highly 

 magnified, bearing comparison with the most skilful masonry of man. 

 From the same, sandy bottom one species picks 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, perforated 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 ' laying ' 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, wiiile forming the walls of its chambers of ordinary sand-grains 

 rather loosely held together, shapes the conical mouths of the suc- 

 cessive chambers by firmly cementing to each other the quartz -grains 

 which border it. To give these actions the vague designation ' in- 

 stinctive ' 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 conceive how so artificial a selection 

 can be made by creatures so simple. 



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. 

 607). Thus, SpiriUina has a minute, spirally convoluted, undivided 

 tube, resembling that of Cornuspira, but having its wall somewhat 

 coarsely perforated by numerous apertures for the emission of pseudo- 

 podia. 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 XIX, figs. 12, 13, U, 15) the mouth is narrowed and 

 prolonged into a tubular neck, giving to the shell the form of a micro- 

 scopic 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 

 Lagena, 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 Nodo- 

 saria (Plate XIX, fig. 16) 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. 17) 

 we have a similar succession of chambers, presenting the characteristic 

 radiate aperture, and often longitudinally ribbed, disposed in a 

 nautiloid spiral. Between Nodosaria and Cristellaria, moreover, 

 there is such a gradational series of connecting forms as shows that 

 no essential difference exists between these two types, and it is a fact 

 of no little interest that some of the simpler of these varietal forms, 



So 2 



