GENUS AMPHISTEGINA. 243 



earlier, so as partially or even entirely to conceal the central boss ; and there is occasionally 

 an apparent interdigitation between the alar lobes of successive wliorls, arising out of diffe- 

 rences in the extent of their central prolongation. This appearance is sometimes seen also 

 on the under surface ; but as the alar lobes of that side much less frequently stop short of 

 the centre than they do on the upper side, those of each whorl usually conceal those of the 

 preceding. — The position of the aperture and the direction of the septal plane in Ampldstegina 

 constitute very marked features of differentiation from the ordinary Nunmiuline type, and of 

 approximation to the Rotaline. For whilst in X/ninimlina and its immediate allies, the septal 

 plane is at right angles to the median plane of the spiral, and the aperture is a narrow fissure 

 between its inner margin and the edge of the penultimate convolution, the septal plane of 

 AnipJmfet/ina is very oblique, making a very obtuse angle with the under surface of the shell 

 nd an acute angle with the upper, and it adheres closely to the penultimate convolution 

 from its margin to its upper umbo, separating from it on the lower side at about one-third 

 of the distance between the margin and the lower umbo, so as to form a much wider and freer 

 opening than exists in the typical Nummuhnes. Thus, whilst the proper Nummuline aperture 

 belongs equally to both sides, the Amphistegine aperture belongs exclusively to the lower side, 

 as in the Rotaline group. The septal plane is differentiated in texture from the ordinary 

 chamber-wall, being destitute (except near its margin) of the fine tubularity by which the 

 latter is characterised ; but it is usually perforated by a few large " orbuline" pores. 



422. The ornamentation of the proper surface of the shell by exogenous deposit consti- 

 tutes a very peculiar feature of this type. The septal bands are often rendered " limbate" 

 by rows of tubercles composed of non-tubular shell-substance ; and a smooth deposit of the 

 like substance along the margin often forms a sort of projecting keel. The general surface fre- 

 quently receives an investment of the like character, which may be composed of very minute, 

 closely-set granules, or of larger and more separated tubercles ; this may cover the whole of 

 each lateral surface, but if restricted to one portion, it is found rather upon the older than 

 upon the newer segments ; and such an investment is so constant upon that portion of the 

 under side which lies nearest to the aperture, that its presence there is one of the ordinary 

 characters of this type, the surface being crowded with granulations which are generally 

 arranged in rows, and sometimes melt into ridges, whose direction is transverse to that of the 

 septal bands. The outer surface of the septal plane is itself very commonly ornamented (as 

 in many Biscorbina) by tubercles, tears, or ridges, of clear substance, which are arranged in 

 lines that pass from the apex to the base of the plane, and which often so project over the 

 low-arched aperture as almost to divide it into a line of pores (fig. 24). In some of the conus 

 shaped fossil Awp/iistef/incs from St. Domingo, the upper (flat) surface presents several large 

 tubercular masses, each about equal to an ordinary upper " boss." 



423. Internal Sfrucfure. — Although most of the principal features of the organization of 

 Amj]Jii!if('(/iiu( may be made out by transparent sections of tlie shell taken in different directions, 

 yetthere are others which such sections altogether fail to elucidate ; and it is fortunate for the 

 true comprehension of these, that we have the additional means of information afforded by 

 " casts" of the interior of the chambers (see p. 10). Turning our attention in the first instance 

 to the vertical section which is represented in Plate XIII, fig. 26, we observe a very marked diffe- 



