2864 Microscopical Society. 



each cell or chamber in this species being subdivided into square compartments by 

 secondary septa, arranged at right angles to the primary ones : the periphery of the 

 horizontal section in this species consists of a network of anastoraozing canals, 

 while the adjacent parietes of the shell are freely supplied with true pseudopodium 

 tubes. 



The next object described was a species of Nonionina from Manilla ; in this shell 

 all the septa are rendered distinctly visible externally, by the marked contrast which 

 exists between their solid and consequently translucent texture, and the more opaque, 

 yellowish-white parietal tissues, in which the foramina are found, also is remarkable 

 for its thickened peripheral margin. In the parietes of this shell we again find a 

 lamellar structure, similar to that occurring in Amphistegina gibbosa ; the greater 

 portion of the shell is perforated with the usual pseudopodium tubes. 



The next group to which attention was directed is that comprehended in 

 D'Orbigny's genus Orbiculina, including 0. complanata, and previously designated 

 Orbitoides and Marginipora, which aTe still regarded by many British zoologists as 

 true Bryozoa. In O. complanata, the central or primordial cell is a large spherical 

 cavity, partially separated from an adjoining one by an imperfect intervening septum : 

 the subsequently added cells are arranged round these two, as a central nucleus : at 

 first they appear as oblique rows, and then as concentric circles, consequently the pri- 

 mordial cell does nol occupy the geometrical centre of the disk. The cells are single, 

 undivided cavities, elongated vertically, and communicating laterally with the conti- 

 guous ones belonging to the same concentric row, by means of large central orifices : 

 they also communicate with those which, though contiguous, belong to different 

 circles, by means of similar orifices ; the apertures are all arranged in the central 

 plane of the organism. From this cause the external margin of the disk has always 

 exhibited a series of marginal apertures, whence the name of Marginipora : the author 

 has not yet been able to detect any superficial orifices or pseudopodium tubes in this 

 object. 



In the section of a large species of Orbiculina, from Tonga, the same general 

 features were observed as in the preceding species, viz., a primordial cell, and three 

 others, round which are other cells ; but instead of the surface exhibiting the closed 

 extremites of a series of oblong cells, as in that species, we here find concentric rows 

 of small oval iossx. These fossae are partially closed inferiorly by a rounded calca- 

 reous body, on each side of which are small circular apertures communicating with 

 the tissues below. In this species, instead of having but one canal connecting each 

 cell with the contiguous one of an adjoining circle, we have here several. The prin- 

 cipal distinction between the internal structure of O. complanata and that from 

 Tonga, is merely such as arises from the multiplication of strictly analagous parts. 

 A considerable difference, however, occurs in the development of the cells in the 

 Tongese species : instead of increasing by slow degrees, and the addition of several 

 rows while they ultimately form a complete circle, we have here a complete circle at 

 once formed around the primordial cell. 



In comparing the development of these Orbiculinas with those of other well-known 

 Foraminifera, we see peculiarities attending the former examples, yet strictly con- 

 forming to one general type : the existence of a central cell, more or less distinct 

 from those by which it is surrounded, has been shown to characterize nearly all the 

 Foraminifera ; on the other hand, the author is not acquainted with any of the Bryozoa 

 whose polypodom exhibits any such structure, and concludes that whilst the struc- 



