82O MICKOSCOPIC FOEMS OF ANIMAL LIFE 



of which many are to be met with on our own shores, but which are 

 more abundant on those of the Mediterranean and especially of the 

 Adriatic, can be traced backwards in geological time at least as far 

 as the Permian epoch. In another genus, Polymorphina, we find 

 the shell to be made up of lageniform chambers arranged in a double 

 series, alternating with each other on the two or more sides of a 

 rectilinear axis ; 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 in the general 

 configuration of the shell, which are indicated by the name it bears. 

 Globigerinida, Returning once again to the simple ' monothala- 

 mous ' condition, we have in Orbulina a minute spherical shell that 

 presents itself in greater or less abundance in deep-sea dredgings, 

 from almost every region of the world a globular chamber with 

 porous walls, but destitute of any general aperture, the office of which 

 is served by a series of larger pores scattered throughout the wall of 

 the sphere. It has been maintained by some that Orbulina is really 

 a detached generative segment of Globigerina, with which it is 

 generally found associated. The shell of Globlgerina consists of an 

 assemblage of nearly spherical chambers (fig. 619), having coarsely 



PIG. 619. Globigerina bulloides as seen in three positions. 



porous walls, and cohering externally into a more or less regular 

 turbinoid spire, each turn of which consists of four chambers pro- 

 gressively increasing in size. These chambers, whose total number 

 seldom exceeds sixteen, may not communicate directly with each 

 other, but open separately into a common ' vestibule ' which occupies 

 the centre of the under side of the spire. This type has attracted 

 great attention, from the extraordinary abundance in which it occurs 

 at great depths over large areas of the ocean bottom. Thus its minute 

 shells have been found to constitute no less than 97 per cent, of 

 the 'ooze ' brought up from depths of from 1,260 to 2,000 fathoms 

 in the middle of the northern parts of the Atlantic Ocean. The 

 younger shells, consisting of from eight to twelve chambers, are 

 thin and smooth, but the older shells are thicker, their surface is 

 raised into ridges that form an hexagonal areolation round the pores 

 (fig. 620) ; and this thickening is shown by examination of thin 

 sections of the shell to be produced by an exogenous deposit around 

 the original chamber wall (corresponding with the ' intermediate 

 skeleton ' of the more complex types), which sometimes contains 

 little flask-shaped cavities filled with sarcode as was first pointed 

 out by the late Dr. Wallich. But the sweeping of the upper waters 



