208 PEOF. AV. J. ^OLL.IS OX SACCA^IMTXA CAETEET, [vol. Ixxvii, 



from, observations made on shells of Sp. vivipara which were ground 

 away on the upper side and on fragments of such shells. These 

 reveal numerous pores, which, when traced inwards, are seen to end 

 blindly, sometimes expanding at their termination into a spherical 

 or more or less irregular pouch-like vesicle (fig. 7, p. 207). They 

 present the appearance of having been produced by some boring 

 parasite. Whether all the pores are of this nature is uncertain. 



There is a close resemblance between the ' pores ' of SjyiriUina 

 and those of Penerojjlis. In some specimens of P. perfusus the 

 ' pseudopores,' as we may term them, are numerous and regularly 

 arranged in rows alternating with the ribs ; in others they are 

 completely absent. Further, an examination of the interior of the 

 shell shows that when present they frequently enlarge at the end 

 into vesicles of precisely the same nature as those of Sp. vivipara. 

 It may be added that mycelium-like threads burrow through the 

 shells both of Spirilliaa and Peneropilis, as they so commonly do 

 in the Foraminifera in general. 



Even more surprising is the structure of the shell, both in itself 

 and its amazing variety. In the simplest case, well exemplified in 

 Sp. ohconica, Sp. infiindihidata. Sp). Jucida, and Sp. vivipara^ the 

 shell is a single homogeneous crystal with, it may be, a few minute 

 grains of calcite sporadically dotted over it, like foreign bodies. 

 The direction of the optic axis differs in different specimens of the 

 same species obtained in one gathering : sometimes it is perpendi- 

 cular to the plane of the spkal, sometimes more nearly parallel to it, 

 and between these extremes it may take any intermediate position. 

 Here then we encounter another excellent example of the indepen- 

 dence of outer form and inner structure in an organic skeleton. 



In Sp. vivipara we find, in addition to this structure, several 

 others ; we meet with forms, for instance, in which fibrils having 

 the optic axis longitudinal make their appearance, and are so 

 arranged that the shell remains illuminated through a whole 

 rotation about its axis : a faint black cross, however, may be 

 detected, and the arrangement of the fibrils is tangential. From 

 this we easily pass to others in which the fibrils are arranged along 

 curved radii, making an angle with the spiral of the shell and giving 

 a spiral cross in the middle which extends over the first whorl : in 

 this case also the optic axis of the fibrils is longitudinal. But by 

 far the most interesting case is that in which the shell consists of 

 an irregular mosaic of crystals. In ordinarj" light this structure is 

 invisible, part of the shell presenting a granular appearance, and 

 part being apparently homogeneous and devoid of granules ; but in 

 polarized light the mosaic is very clearly disjDlayed (PL YII, 

 figs. 7 & 7«). The thickness of the wall as seen in optical section 

 is between "02 and "03 mm. 



In SpirilJina limhata the shell is more granular than fibrous, and 

 remains illuminated, except for some irregular areas, throughout a 

 complete rotation between crossed nicols ; yet in places a negative 

 fibril, tangential for the greater part, extends so far round a whorl 

 that its optic axis passes from tangential to radial. 



