10G 



intersected by numerous bluish, translucent lines, which, though 

 somewhat irregular in their external aspect, exhibit a disposition to 

 radiation from the centre towards the circumference of each lateral 

 half of the shell. 



On making a horizontal section of this species at its greatest diame- 

 ter, I obtained the appearance presented by fig. 1, PI. 17 ; and on pre- 

 paring a section in the opposite or vertical direction, its aspect was 

 that of fig. 2. From the first of these we see that, in its principal 

 features, this species exhibits the ordinary contour seen in so many 

 of the Foraminifera ; viz., a spiral arrangement of cells, separated 

 from each other by calcareous septa. Fig. 2 shows that the object 

 is not only inequilateral, its inner convolutions especially being 

 more convex on one surface than on the other, but that the convolu- 

 tions are not all on one plane ; the earliest formed spirals being on 

 a lower level than those of a more recent growth. Hence it is im- 

 possible to make a horizontal section of this species, in which the 

 incision shall pass through the middle of each convolution : some 

 of them must be either above or below it. From this cause, fig. 1, 

 which represents a section made in the direction of a line drawn 

 from fig. 2, a, to 2, a, passes above the median plane of the innermost 

 convolutions ; and hence the substance of the shell, being cut 

 through obliquely, has at this point a somewhat different aspect to 

 that of the outermost convolution. 



In the arrangement of the septa, as seen in fig. 1, Amphistegina 

 differs but little from the ordinary type of the rotuline forms ; the 

 septa being as usual, convex anteriorly and concave posteriorly ; 

 whilst above and below this median line the segments of the soft 

 animal and their corresponding septa are prolonged in the direction 

 of the centre of each lateral half of the organism. One aperture 

 exists (1, b) in each septum, as in Rotulina and Nonionina, communi- 

 cating between contiguous chambers, the septum being often thick- 

 ened at that point, especially on its anterior surface. Excepting 

 in the case of two or three of the septa, the section, in this in- 

 stance, has not crossed the aperture, but passed a little above it. 



This specimen exhibits the existence of numerous calcareous 

 papilla? and projecting pillars, which are thickly dispersed over the 

 inner floor of each cell (1, c) and the anterior surface of each septum 

 (1, c), but which do not exist on the posterior concave surface of the 

 septa or on the glossy external parietes of the shell, excepting in 

 the immediate vicinity of the oral orifice in the anterior septum of 

 the shell. The fact that the greatest portion of the exterior is so 



