116 



section. They are single, undivided cavities (9, a), elongated verti- 

 cally, and communicating laterally with the contiguous ones belong- 

 ing to the same concentric row, by means of large central orifices 

 (8, d, and 9, b, d). They also communicate with those which, though 

 contiguous, belong to different circles, by means of similar orifices 

 (9, c). The margin of each of these communicating orifices is usually 

 thickened, and in frequent instances, as at 9, d, the orifice is not a 

 mere aperture in the septum, but assumes a somewhat spiral or con- 

 voluted form, owing to the corkscrew-like twist given to the septum 

 at this point. Fig. 10 represents a horizontal section of a central 

 primordial cell of this species, with a few of its earliest additional 

 growths. This example shows, what has already been alluded to, 

 that the cells of the organism, when viewed in their superficial aspect, 

 do not at once form complete concentric circles. In the specimen 

 under consideration we observe, first the large cavity (10, a), then 

 the second portion of the same (10, b), separated from a by the 

 imperfect septum (10, c). Upon this is built another large, but 

 complete cell (10, d), communicating with & by a single small aper- 

 ture in the septum which divides them. We then have an additional 

 row of four cells (10, e e), nearly all of which communicate as before 

 with the preceding cell (d) through corresponding apertures in the 

 septum. In the next row (10, //') we have also four similar cells, 

 and in the succeeding one (g g') their number has increased to seven. 

 Soon after the development of this row, the newer series has begun 

 to bend round the point g, covering in succession the extremities of 

 the cells /', e' and d; whilst at their opposite portion (g) similar en- 

 croachments are made upon the periphery of the primordial cell, 

 until at length, by the contact of the two extremities, complete con- 

 centric circles are formed, and continue to be so until the animal 

 arrives at maturity. At the same time that they are thus arranged, 

 they also observe the order seen in the disk of a Coscinodiscus, or 

 the back of an engine -turned watch. The specimen (fig. 10) also 

 shows us how irregularly the communicating apertures are distri- 

 buted, though they are all arranged in the central plane of the orga- 

 nism. Sometimes they are isolated, as at i, merely connecting two 

 adjacent cells together, belonging either to the same or to contiguous 

 circles ; at others they connect three such cells, two belonging to the 

 same row, and one belonging to a more external series, as at 10, k. 

 The latter appears to be the form seen in the vertical section at 9, c. 

 From this cause the external margin of the disk has always exhibited 

 a series of marginal apertures, whence the old name of Marginipora. 

 I have not been able to detect any superficial orifices or pseu- 



