244 FAMILY NUMMULINIDA. 



rence in tlie convexity of the upper and the under surfaces, and in the position of the primordial 

 chamber in respect to each ; and this difference is due in part to the gradual declination of 

 the spire along the plane traversed by the line g, h, and in part to the excess in the growth of 

 all the parts below that line as compared with that of the parts above it, this being apparent 

 not merely in the size of the cavities, but also in the thickness of the layers of shell by which 

 they are separated. It will be observed, moreover, that whilst the alar lobes {a, a) of the 

 upper sides thin-out rapidly as they approach the centre of each convolution, those of the lower 

 side [a, a^) thin-out much more slowly, and are often continued as far as its centre. The 

 shell is finely-tubular, like that of Eotalia, Operculum, and NiimmitHna ; and is composed 

 of several lamellre superposed one on the other, the tubuli being continuous throughout. 

 At the umbonal portion ib, b'^,) of each surface, however, the tubuli are deficient, the 

 shell-substance being there quite pellucid ; and the same condition usually presents itself at 

 the margin {c, c^ c", c') of each convolution. A very considerable difference presents itself 

 between the thickness of the chamber-walls of the last convolution {e, e^) and that of the pre- 

 ceding {d, (P) ; and this will presently be found to depend, as in Calcarina, upon an exogenous 

 shell-growth superimposed upon the proper chamber-walls ; such increase, however, being made 

 by the successive addition of laminae exactly corresponding in structure to the original one, and 

 so perfectly continuous with it and with each other that the pseudopodial tubuli may be traced 

 ■without the least interruption througli their entire thickness. This section further shows the 

 position of the aperture (/,/^) entirely below the marginal plane of each convolution ; and it 

 exhibits also the projection of the tubercles formed by exogenous deposit upon the older parts 

 of the surface, into the cavities of the chambers by which that surface subsequently comes to 

 be invested. At i is shown an apparent subdivision of the outer wall of tlie convohition, so 

 as to form a small additional chamber outside the principal one ; but it will be found, when 

 the vertical section is compared with the horizontal (fig. 25), that this apparent subdivision 

 is due to the remarkable backward curvature of the septa, whicli often causes sections that 

 are taken in a radial direction to traverse the inner portion of one chamber, and the outer 

 portion of another chamber of the same convolution. 



424. Owing to the turbinoid form of the spire, a section through any horizontal plane 

 cannot traverse more than one convolution in its median plane, and must pass either above or 

 below the median plane of the rest. The horizontal section shown in fig. 25, passes through 

 the median plane of the outer convolution ; consequently it lays open the penultimate convolu- 

 tion along its under side ; and it passes altogether beneath the interior convolutions. The 

 remarkable backward curvature of the septa, by wliich every septum glides (so to speak) into 

 the periphery of the convolution, along which it is continued, is a very distinctive feature in 

 this type ; the continuity of the shelly lamina formed at each increase over the whole of the 

 surface previously exposed, being made evident by the regularly progressive increase in the 

 thickness of the peripheral layer from the newest chamber to the earliest of the same convolu- 

 tion. It is a very peculiar feature of this type, that its septal lamellae are not double, as in 

 the proper Nummulines, but single, as in the lower Rotalines ; so that each chamber is a 

 " tent " set upon the exterior of the preceding. At the inner margin of certain septa, we 

 see that they stop short of the wall of the previous convolution ; this being where the 

 plane of the section has happened to pass through the aperture. Tiie anterior surfaces of the 



