PRINCIPLES OF CLASSIFICATION. 45 



though often very deep, never extends through the whole thickness of the shell. Some kind 

 of inequality of that surface, indeed, is extremely common in the shells of the porcellanous 

 Foraminifera. Very frequently it presents an alternation of ridges and furrows, such as dis- 

 tinguishes certain varieties o^MilioJa (Plate VI, figs. 3, 4) ; and this alternation is so regular and 

 constant in Prawo/V/s (Plate VII, fig. 18), and in the first-formed segments of Vertebralina 

 (Plate V, figs. 17 — 25), as to be peculiarly characteristic of those types. In other instances, 

 especially among the Miliohc, we find the surface marked by depressions which may vary 

 greatly in size and in arrangement (Plate VI, figs. 13, 14), being sometimes minute puncta- 

 tions, in other cases being lai'ge areolae ; being sometimes scattered with no apparent regularity, 

 in other instances disposed with the most exact symmetry. But no difference of texture 

 accompanies any of these inequalities of the surface, the raised and depressed portions being 

 alike homogeneous. 



58. In the shells of the vitreous or hyaline type, on the other hand, the proper shell- 

 substance has an almost glassy transparency ; which is shown by it alike in the thin natural 

 laminas of young specimens, and in artificially-prepared sections of such as are thicker and 

 older. It is usually colourless, even when (as is often the case with Rotalia) the substance 

 of the animal is deeply coloured. In certain aberrant forms of the Rotaline type, 

 however, we shall see that the shell is commonly, like the animal body, of a rich crimson hue. 

 But notwithstanding the transparence of their substance, these shells derive an adventitious 

 opacity from being channeled out more or less minutely by tubular perforations, which, when 

 occupied either by air or by any substance having a refractive power different from that of 

 the surrounding shell, interfere with its power of transmitting light, and cause it to reflect a 

 large part of that which falls upon it. Their effect varies, however, according to their degree 

 of minuteness and the closeness of their arrangement. Thus, in Rotalia, in which they are 

 commonly almost 1 -3000th of an inch in diameter and somewhat more than that apart 

 from one another (Plate XIII, fig. 1, a), the hyaline transparence of the thin shell makes 

 itself apparent between them, and imparts to its entire surface a vitreous aspect. 

 In Opercidina and Cycloclypem, on the other hand, in which the average diameter of 

 the tubuli does not exceed l-10,000th of an inch, and their distance from each other is 

 not much greater, every part of the shell that is traversed by them presents a semi- 

 opacity, which only disappears when extremely thin sections are made in a direction 

 exactly transverse to the axis of the tubes, so as to enable the transparence of the intervening 

 substance to display itself without interruption (Plate XIX, fig. 4). It often happens, how- 

 ever, that certain parts of the shell are left unchanneled by these tubuli ; and such are at once 

 distinguished, even under a low magnifying power, by the readiness with which they allow 

 transmitted light to pass through them (Plate XVII, figs. 12, 13, aa, da, U\ and by the peculiar 

 vitreous lustre they exhibit when light is made to fall obliquely upon their surface. In shells 

 formed upon this type we frequently find that the surface presents either bands or spots which 

 are thus distinguished ; the non-tubular bands usually marking the positions of the septa (Plate 

 XVII, fig. 1), and being sometimes raised into ridges, though in other instances they are either 

 level or somewhat depressed, whilst the non-tubular spots may occur on any part of the 

 surface, and are most commonly raised into tubercles (Plate XV, fig. 5, and Plate XIX, fig. 5), 

 which sometimes attain a size and number sufficient to give a very distinctive character to the 



