34 DE. CAEPENTEE'S EESEAECHES ON THE FOEAMINIFEEA. 



already been made, which causes them to unite at intervals with the spiral lamina that 

 is subsequently formed over them ; so that they are marked on its external surface by the 

 non-tubularity of its substance in those parts which immediately overlie them. 



169. Allowance being made for the differences arising out of this diversity in their 

 plan of conformation, we find an extraordinary resemblance in the general organization 

 of Amphistegina Cumingii to that of the Operculina already described, as will be appa- 

 rent from a comparison of figs. 5, 6, Plate VI. to figs. 8, 10, Plate IV. of Operculina, 

 Hence it will be quite unnecessary to enter into any minute description of its structure. 

 In regard to the mode of communication between the chambers, by one principal 

 orifice in the form of a narrow slit along the inner margin of each septum, with a 

 variable number of secondary apertures irregularly disposed in different parts of the 

 septal plane, there is an absolute identity with Operculina ; as there is also in the dis- 

 tribution of the interseptal passages between the two layers which each septum here 

 unmistakeably exhibits. The marginal cord, likewise, exhibits the same peculiarity 

 of organization as in Operculina ; being channeled out by freely inosculating passages 

 which communicate with those of the interseptal system (Plate VI. fig. 6). This mar- 

 ginal system of passages is even more developed than in Operculina ; and a study of its 

 distribution will, I think, remove all doubt as to the correctness of my interpretation of 

 the appearances often presented by sections of the marginal cord in that genus (^[ 156). 

 There is this difference, however, in the relation of the interseptal to the marginal 

 system of canals in these two types, that the interseptal canals of Amphistegina appear 

 to take their origin directly from some of the large superficial passages of the marginal 

 cord on which the septum abuts, instead of branching from a pair of regular spiral canals 

 as in Operculina, no distinct evidence of such canals having here presented itself to me. 



170. The relationship of Amphistegina Cumingii to those forms of Nummulites in 

 which the alar prolongations of the chambers of each whorl are continued, with centri- 

 petal prolongations of the septa, over the whole surface of the penultimate whorl, is 

 obviously extremely close; in fact, I can discern no distinguishing character between 

 these two types, except that which is afforded by the tendency of the last turn of the 

 spire of Amphistegina to open out, whilst that of Nummulites closes in. The double- 

 coned axis of non-tubular substance would seem to be a distinctive character of Amphi- 

 stegina ; but I am not inclined to lay much stress upon it, as it presents itself in very 

 different proportions in different individuals. 



171. Whilst the type which I have now been describing is thus closely related to the 

 highest because most specialized forms of Foraminiferous organization, it is singular 

 that the smaller species to which I have referred under the name of A. gibbosa should 

 present such a marked inferiority of conformation. Its spiral lamina is as minutely 

 tubular as that of its congener ; but I have not been able to detect the least trace of a 

 system of interseptal canals ; and it is especially to be noticed, that the marginal cord, 

 which exhibits such a high development of the canal-system in A. Cumingii, consists in 

 A. gibbosa of homogeneous non-tubular shell-substance, without any vestige of canals, 



