GENUS AMPHISTEGINA. 245 



septa, like the margins of the enclosed whorls, are seen to be studded with large tubercles, 

 which project into the cavities of the chambers. In the central portion of the section arc seen 

 the indications of the converging prolongations of the septa, which are interposed between 

 the alar lobes of the under surface. In those portions of tlie inferior chamber-wall which have 

 not been removed by the section, we see the general surface minutely studded with the 

 orifices of the pseudopodial tubuli, which are considerably larger on the inner surface than 

 they are on the exterior ; these, however, are wanting in tiiose parts that underlie the 

 tubercles of the surface, which are always composed of non-tubular substance (fig. 25, a). 



425. Nothing is shown either in vertical or in horizontal sections to explain that peculiar 

 disposition of the septal lines which has been already stated to present itself upon the under 

 surface of the shell; and this might have remained without adequate elucidation, but for the 

 fortunate accident which has placed in the possession of Messrs. Parker and Rupert Jones the 

 means of obtaining internal casts of the cavities of this shell in silicate of iron. These casts 

 so perfectly represent the form of the sarcodal segments which occupied the interior of the 

 chambers, that their surfaces are rendered hispid by the minute projections which passed into 

 the entrances of the tubuli of the chamber-walls. On looking at the upper side of one of these 

 casts (fig. 27), we see, as we should expect, the "alar lobes '' prolonged inwards from the anterior 

 portions of the peripheral lobes or "bodies" of the successive segments, and gradually narrowing 

 towards the centre, which they do not quite reach. But on looking at the under side (fig. 28), 

 we observe that the peripheral bodies of the segments only fold over that surface for a 

 short distance, and then suddenly round themselves off; whilst the place of their alar pro- 

 longations is occupied by a set of " astral lobes," which interdigitate in a very peculiar 

 manner with the bodies of the segments, and send extensions peripherally to the very margin 

 of the spire. Although, at first sight, these astral lobes appear to be altogether disconnected 

 from the bodies of the principal segments, yet, on a careful scrutiny of the most perfect casts, 

 we find that the peripheral extension of each astral lobe is continuous with the retral extension 

 of the body of the segment that lies in front of it, as shown at a, u ; this continuity being 

 sometimes established by a thick neck, and sometimes by a very slender peduncle. Hence it 

 is obvious that these " astral lobes " are nothing else than the alar prolongations of the 

 chambers on the under side, each being pinched oif (so to speak) from its principal segment 

 by the intervention of a septum, and being also somewhat displaced backwards. 



426. We have seen that the surface of the shell in the neighbourhood of the aperture 

 so commonly becomes studded with tubercles of exogenous deposit, that their presence may 

 almost be regarded as among the characteristics of this genus ; and since, as each new chamber 

 is formed, the tuberculated surface is received into it, and a new exogenous deposit takes place 

 beyond, every chamber will thus exhibit a projection of these tubercles into its interior. 

 Now it commonly happens that this deposit extends from the margin towards the centre of the 

 shell, so that the floors of the alar prolongations of the chambers become studded with 

 tubercles ; these are sometimes small and closely set without any particular arrangement, 

 whilst in other instances they are fewer and more prominent, in which last case they are very 

 commonly arranged in radial lines. These varieties are very beautifully marked in the 

 "casts" of the alar and astral lobes. In some instances the inner surface of these lobes is 



