68 THE MICROSCOPE AND ITS REVELATIONS. 



binoid spiral, all the convolutions are usually visible, at least on one side 

 (figs. 15, 17, 18), but among the Nautiloid tribes it more frequently hap- 

 pens that the last-formed whorl incloses the preceding to such an extent 

 that they are scarcely, or not all, visible externally, as is the case in Cris- 

 tellaria (fig. 11), Polystomella (fig. 16), and Nonionina (fig. 19). The 

 turbinoid spire may coil so rapidly round an elongated axis, that the 

 number of chambers in each turn is very small; thus in Globigerina (fig. 

 12) there are usually only four; and in Valvulina the regular number 

 is only three. Thus we are led to the Userial arrangement of the cham- 

 bers which is characteristic of the Textularian group (fig. 14); in which 

 we find the chambers arranged in two rows, each chamber communicat- 

 ing with that above and that below it on the opposite side, without 

 any direct communication with the chamber of its own side, as will be 

 understood by reference to Fig. 328, A, which shows a ' cast ' of the sar- 

 code body of the animal. On the other hand, we find in the nautiloid 

 spire a tendency to pass (by a curious transitional form to be presently 

 described, 464) into the cyclical mode of growth; in which the original 

 segment, instead of budding-forth on one side only, develops gemmce all 

 round, so that a ring of small chambers (or chamberlets) is formed around 

 the primordial chamber, and this in its turn surrounds itself after the 

 like fashion with another ring; and by successive repetitions of the same 

 process the shell co'mes to have form of a disc made up of a great num- 

 ber of concentric rings, as we see in Orbitolites (Fig. 316) and in Cyclo- 

 clypeus (Plate xvi., fig. 1). 



458. These and other differences in the plan of growth were made by 

 M. D'Orbigny the foundation of his Classification of this group, which, 

 though at one time generally accepted, has now been abandoned 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 condi- 

 tion of the Animal, and are thus indicative of the real affinities of the 

 several groups which they serve to distinguish. The most important of 

 these characters will now be noticed. 1 



459. Two very distinct types of Shell-structure prevail among ordi- 

 nary 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 transmitted 

 light, the opacity gives place to a rich brown or amber color, which in a 

 few instances is tinged with crimson. No structure of any description 

 can be detected in this kind of shell -substance, which is apparently 

 homogeneous throughout. Although the shells of this i porcellanous ' 

 type often present the appearance of being perforated with foramina, yet 

 this appearance is illusory, being due to a mere ' pitting' of the external 

 surface, which, 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 ' Fora- 



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

 the Study of the Foraminifera," 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. 



