802 MICROSCOPIC FORMS OF ANIMAL LIFE 



instance, the form which has been designated as Spiroloculina. This 

 shell is a spiral, elongated in the direction of one of its diameters, 

 and having at 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 often 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. Xow it is a very common habit in the milioline 

 type for the chambers of the later convolutions to extend themselves 

 over those of the earlier, so MS to conceal them more or less com 

 pletely ; and this they very commonly do somewhat unequally, so that 

 more of the earlier chambers are visible 011 one side than on the 

 other. Miliolw thus modified (tig. 1, PI. XVIII) have received the 

 names of Quinqueloculina 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 extended, entirely destroys their value as differ- 

 ential characters, and the term Miliolina is now more frequently 

 applied to them collectively. Sometimes, on the other hand, 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 Siloculina, 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 face> 

 the observer. It is very common in milioline shells for the external 

 surface to present a ' pitting,' more or less deep, a ridge-aiid- furrow 

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

 have been used for the characterisation of species. Not only, how 

 ever, 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 honey- 

 combed ; so that here, again, the inconstancy of these differences 

 deprives them of much of their value as distinctive characters. 



An interesting illustration of the tendency to dimorphism 

 amongst the Foraminifera has been observed by MM. Munier 

 Chalmas and Schlumberger ! in the .structure of the shells of this 

 -roup. They find that while two forms, which they distinguish as 

 form A and form B, are similar externally they differ in internal 

 structure, form B having its initial chamber much smaller than that 

 of form A. and this ' microsphere ' is followed by a larger number of 

 chambers than is the : megasphere ' of form A. What this difference 

 signifies it is at present impossible to say. but it has been suggested 

 that it may be one of sexual character, or, better, of a series in a cycle 

 of general ions. The observations of the French naturalists referred 

 to open out a new field of inquiry, and one which is enjoying the 

 attention of seseral \vorkers in this department of research. - 



1 11/i/lr/in iS'oc. Geol. ser. iii. vol. xiii. j>. '27:',. 



3 Gf. J. J. Lister in Phil. Trans. l:;r, B (IS'.c.i, p. 401, and F. St-haudimi, ' Ueber 

 den Dimorphismus der Foraminifereii,' S.B. Ges. Natitrf. Jlciiin, l.s!ir>, \>. 87. 



