118 



latter portion we perceive that the species differs considerably from 

 O. complanata. Instead of the surface exhibiting the closed extre- 

 mities of a series of oblong cells, as in that species, we here find 

 concentric rows of small oval fossae or pits (12, a, and 13, c).* These 

 fosses are partially closed inferiorly by a rounded calcareous body 

 (12, b, and 13, d), on each side of which are small circular apertures 

 (12, h, and 18, e,/), communicating with the tissues below. 



The enlarged representation (12) of the vertical section gives us 

 the clew to all this. We see that the superficial fossae do not corre- 

 spond in their position with the subjacent cells, but are arranged in 

 rows immediately above the septa which separate the respective 

 circles of cells from one another; the upper surfaces of the septa 

 constituting the bases (12, b, and 13, d) of the fossee ; whilst the two 

 canals (12, h, and 13, e,f) communicate right and left with the con- 

 tiguous cells belonging to two adjoining circles : the real exter- 

 nal coverings of the cells are the calcareous portions represented by 

 12, d, and 13, g. We observe that all these structural details are 

 repeated on the opposite surface of the disk (12, e). 



On descending below the surface, and examining the interior of 

 the organism, we find that it appears to present a very considerable 

 amount of difference from the corresponding portions of O. compla- 

 nata (fig. 9). A closer examination, however, will show that this 

 difference is more apparent than real. The dark, shaded portions 

 (12, 6 b') are in reality nothing more than the septa dividing the 

 cells (12, g), and correspond with the analogous septa, fig. 9, e. 

 But in this species, instead of having only one canal connecting each 

 cell with the contiguous one of an adjoining circle, as in fig. 9, c, we 

 here have several (12, c c). And in a similar way, instead of having 

 only one corresponding lateral communication connecting each cell 

 with the contiguous ones in the same circle, as at 9, b, we here find 

 a corresponding increase in their number (12,//'). Thus the dis- 

 tinction between the internal structure of 0. complanata and that 

 from Tonga is merely such as arises from the multiplication of 

 strictly analogous parts in the latter instance. I may remark with 

 reference to the lateral apertures (fig. 12,//'), that whilst the distri- 

 bution of those which occupy the central portions of the organism 

 (/) is exceedingly irregular, those which are nearest to each surface, 

 (/') and contiguous to the superficial fossee, are just the reverse. In 

 fig. 1 1 it will be seen that a row of them {d) may be readily traced 

 round the entire circumference of the section. Thus we find that in 



* See Appendix B. 



