FORAMINIFERA AND RAUIOLARIA. 71 



changed from a circle to a long narrow slit. Among the commonest of 

 the Foraminifera, and abounding near the shores of almost every sea, are 

 some forms of the Milioline type, so named from the resemblance of 

 some of their minute fossilized forms (of which enormous beds of lime- 

 stone in the neighborhood of Paris are almost entirely composed) to mil- 

 let-seeds. The peculiar mode of growth by which these are characterized, 

 will be best understood by examining in the first instance the form which 

 has been designated as Spirolpculina (Plate xv., fig. 2). This shell is a 

 spiral, elongated in the direction of one of its diameters, and having in 

 each turn a contraction at either end of that diameter, which partially 

 divides each convolution into two chambers; the separation between the 

 consecutive .chambers is made more complete by a peculiar projection 

 from the inner side of the cavity, known as the 'tongue' or 'valve,' 

 which may be considered as an imperfect septum; of this a characteristic 

 example is shown in the upper part of fig. 4. Now it is a very general 

 habit in the Milioline type, for the chambers of the later convolutions to 

 extend themselves over those of the 'earlier, so as to conceal them more 

 or less completely; and this they very commonly do somewhat unequally, 

 so that more of the earlier chambers are visible on one side than on the 

 other. Miliolce thus modified (fig. 3) have received the names of Quin- 

 queloculina and Triloculina according to the number of chambers visible 

 externally; but the extreme inconstancy which is found -to mark such 

 distinctions, when the comparison of specimens has been sufficiently ex- 

 tended, entirely destroys their value as differential characters. Some- 

 times the earlier convolutions are so completely concealed by the later, 

 that only the two chambers of the last turn are visible externally; and in 

 this type, which has been designated Biloculin*, there is often such an 

 increase in the breadth of the chambers as altogether changes the usual 

 proportions of the shell, which has almost the shape of an egg when so 

 placed that either the last or the penultimate chamber faces the observer 

 (Plate xv., fig. 4). It is very common in Milioline shells for the exter- 

 nal surface to present a ' pitting/ more or less deep, a ridge-and-furrow 

 arrangement (fig. 3), or a honeycomb division; and these diversities have 

 been used for the characterization of species. Not only, however, may 

 every intermediate gradation be met-with between the most strongly 

 marked forms, but it is not at all uncommon to find the surface smooth 

 on some parts, whilst other parts of the surface in the same shell are 

 deeply pitted or strongly ribbed or honeycombed; so that here again the 

 inconstancy of these differences deprives them of all value as distinctive 

 characters. 



^ 463. Reverting again to the primitive type presented in the simple 

 spiral of Cornuspira, we find the most complete development of it in 

 Peneroplis (Plate xv., fig. 5), a very beautiful form, which, although 

 very rare on our own coasts, is one of the commonest of all Foraminifera 

 in the shore-sands and shallow water dredgings of the warmer regions of 

 every part of the globe. This is a nautiloid shell, of which the spire 

 flattens itself out as it advances in growth; it is marked externally by a 

 series of transverse bands, which indicate the position of the internal 

 septa that divide the cavity into chambers; and these chambers commu- 

 nicate with each other by numerous minute pores traversing each of the 

 septa, and giving passage to threads of sarcode that connect the seg- 

 ments of the body. At a is shown the ( septal plane ' closing in the 

 last-formed chamber, with its single row of pores through which the 

 pseudopodial filaments extend themselves into the surrounding medium. 



