52 SIDERASTREA RADIANS. 



arrangement of the septa in groups of three is continued all the way round, 

 and the hexameral plan is altogether obscured. Such an interpretation of 

 the septal plan is at variance with what is established above for the Jamaica 

 representatives of the same species. 



As seen at the surface of a colony the septal edges extend almost hori- 

 zontally for a short distance from the periphery, and are then inclined down- 

 ward and inward somewhat sharply to meet the columella which forms the 

 floor of the middle of the calice. The actual depth of the calice varies some- 

 what in different colonies, but is usually about 2 mm. 



SYNAPTICULA. 



Viewed with a lens from above, the septa are seen to be joined to one 

 another laterally by thick transverse bars the synapticula. In any trans- 

 verse section of a calice one to three, rarely four, synapticula are seen crossing 

 the space between every two adjacent septa, but are limited in their distribu- 

 tion to the peripheral half of the calice (plate 10, fig. 64). In a view of the 

 lateral surface of the larger septa they are found to be arranged in two or 

 three, rarely four, somewhat irregular vertical rows, each synapticulum being 

 circular or oval in section, and projecting at right angles from the septal 

 surface (plate 10, fig. 63). The presence of so many connections between the 

 septa gives a porous or reticular character to the more peripheral part of the 

 calice. 



The synapticula are formed by the ultimate fusion across an interseptal 

 loculus of two opposite granulations on the faces of adjacent septa, the calca- 

 reous matter being deposited in a manner similar to that of other parts of the 

 corallite. In Siderastrea it is only the granules in restricted spots on the 

 septal faces which become thus enlarged. The synapticula have a limited 

 distribution, and the other granulations on the septal face remain compara- 

 tively small, there being no intermediate examples. It is clear that in the 

 course of their growth the enlarging granules must first indent the skeleto- 

 trophic tissues which line them, then bring the two opposite layers together, 

 and finally perforate them, as shown on plate 6, fig. 34 ; also, as described on 

 p. 28, the mesentery inclosed within the loculus is perforated at the same time. 



Sections through the synapticula show that they are formed as lateral 

 processes of the septa, either by an extension of the septal trabeculae or from 

 a center of calcification of their own. In the former case a boundary in the 

 middle of the synapticulum indicates where the bundles of fibro-crystals from 

 one septum have met those from the opposite septum. In the other case it 



