242 FAMILY NUMMULINIDA. 



421. Krteriial Cliaracters. — The shell of Ampldstegina closely corresponds in external 

 form with that of Nmuiudlna ; but it is only for some of the smaller Nummulines that Am- 

 jjliisfi'f/ina could be mistaken, since its diameter seldom exceeds l-8th of an inch, which is 

 not uncommon among the larger RotaUnes. When viewed from either side (Plate XIII, 

 figs. 22, 23), it presents an almost exact circular outline, its spiral growth not much affecting 

 the regularity of the curvature of its margin ; this is partly due to the fact that its chambers 

 are very numerous, and the increase of their size p^radual, and partly to the peculiar direc- 

 tion of its septa (iig. 25). In certain varieties, however, the later chambers thin-out and 

 extend themselves rather suddenly, so that the primordial segment loses its central position, 

 and the entire shell becomes ear-shaped. When viewed edgeways, the shell presents a form 

 somewhat resembling that of the crystalline lens in the unequal curvature of its two surfaces, 

 but having a more acute margin, as is well seen in vertical section (fig. 2G). The under 

 side, that is, the one on which the aperture presents itself, is normally the most prominent ; 

 but frequently there is very little diflTerence between the curvature of the two faces ; and some- 

 times even it happens that the general convexity is greatest on the upper side, whilst the 

 surface of the shell so sinks in around the umbilical " boss" on the lower side, as to render it 

 " circumvallate." On the other hand, the upper surface may be nearly flat, whilst the um- 

 bilical portion of the lower surface becomes very prominent ; so that the general form of the 

 shell, like that of Botalia Schroeteriana, comes to resemble that of a Conns. Occasionally, though 

 rarely, the shell is more or less curled or twisted, and may even become saddle-shaped, like 

 certain individuals among Nummidina and Orbitoides. As, in conformity with the general plan 

 of construction of the Nwmmulinida (^ 414), each convolution normally invests the whole of 

 the preceding, only the last convolution for the most part is visible on the surface of the 

 shell. On looking at its upper side (fig. 22), we see the subjacent septa marked out by lines 

 of a different texture from the rest of the shell, which proceed in a nearly radial direction 

 (though slightly curving forwards) until within a short distance of the margin, and then 

 suddenly bend backwards. On the under side, however, the lines which radiate from 

 the centre are interrupted at a short distance from the margin (fig. 23), returning as 

 it were upon themselves, whilst the marginal backward-curving lines in Uke manner 

 seem to return upon ;'//««selves, thus apparently marking out two sets of chambers 

 which slightly iuterdigitate with one another. The centre of each surface is occupied 

 by a large "boss " of non-tubular shell-substance, which is so pellucid as to allow 

 the subjacent structure to be distinctly seen through it. The contour of this boss is com- 

 monly " flush" with the general curvature on the upper side (fig. 26, h), whilst on the low^er 

 side {1}) it is more prominent. When a vertical section of the shell is examined, each boss 

 is seen to have its origin from the surface of the primordial chamber, and to increase in 

 diameter with every new overgrowth which this receives, so that it comes to acquire a pretty 

 regular conical shape. Hence, as the central terminations of the chambers of each whorl 

 usually stop sliort of those of the preceding whorl, the. latter can be seen to project beneath 

 the former, so that the whole series of these central terminations may be traced inwards 

 from those of the latest to those of the earliest convolution. This is more constantly to be 

 seen on the upper surface than on the lower ; as it is on the former that the alar prolonga- 

 tions of the later whorls stop short at the greatest distance from the centre. Sometimes, 

 however, the alar lobes of the later segments extend nearer to the centre than do those of the 



