EOEAMINIFERA 799 



* 



M. d'Orbigny the foundation of his classification of this group, 

 which, though at one time generally accepted, has now been aban- 

 doned by most of those who have occupied themselves in the study 

 of the Foraminifera. For it has come to be generally admitted that 

 ' plan of growth ' is a character of very subordinate importance 

 among the Foraminifera. so that any classification which is primarily 

 based upon it must necessarily be altogether unnatural, those 

 characters being of primary importance which have an immediate 

 and direct relation to the physiological condition of the animal, 

 and are thus indicative of the rei*l affinities of the several groups 

 which they serve to distinguish. The most important of these 

 characters will now be noticed. 1 



Two very distinct types of shell structure prevail among ordinary 

 Foraminifera namely, the porcellanous and the hyaline or vitreous. 

 The shell of the former, when viewed by reflected light, presents an 

 opaque- white aspect which bears a strong resemblance to porcelain ; 

 but when thin natural or artificial laminae of it are viewed by trans- 

 mitted light the opacity gives place to a rich brown or amber 

 colour, which in a few instances is tinged with crimson. No 

 structure of any description can be detected in this kind of shell sub- 

 stance, which is apparently homogeneous throughout. Although 

 the shells of this 'porcellanous' type often present the appearance 

 of being perforated with foramina, yet this appearance is illusnrv. 

 being due to a mere 'pitting' of the external surface, which, though 

 oft ( j n 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, one of the 

 most frequent forms of it being a regular alternation of ridges and 

 furrows, such as is occasionally seen in Miliola, but which is an 

 almost constant characteristic of Peneroplis (fig. 606). But no 

 difference of texture accompanies either this or any other kind of 

 inequality of surface, the raised and depressed portions being alike 

 homogeneous. In the shells of the vitreous or hyaline type, on the 

 other hand, the proper shell substance has an almost glassy trans- 

 parence. which is shown by it alike in thin natural lamellae and in 

 artificially prepared specimens of such as are thicker and older. It 

 is usually colourless, even when (as in the case with many RotaUimi') 

 the substance of the animal is deeply coloured ; but in some few- 

 species, such as Globig&rina rubra, Truncatulina rosea, and Polytreina 

 Hthnaceum, the shell is commonly, like the animal body, of a more 

 or less deep crimson hue. All the shells of the hyaline type are 

 beset more or less closely \\ti\\tulidarperforatioTts. which pass directly, 

 and (in general) without any subdivision, from one surface to the 

 other. These tubuli are in some instances sufficiently coarse for 

 their orifices to be distinguished with a low magnifying power as 

 punctations ' on the surface of the shell, as is shown in fig. 607 ; 

 whilst in other cases they are so minute as only to be discernible in 

 thin sections seen by transmitted light under a higher magnifying 



1 This subject will be found amply discussed iu the Author's Introduction to the 

 Sfudi/ of tli e Fora / in if era, published by the Ray Society, to which work he would 

 refer such of his readers as may desire more detailed information in regard to it. 



