52 W. G. RIDEWOOD. 



In specimen D, as in specimen 0, the manner in which the coenoecium of the 

 polyzoon in this case Menipea has become embedded in the test of the 

 Cephalodiscus gives one a good idea of the profusion with which this material is 

 secreted. 



The shape of the tubarium of specimen G differs materially from that of the type 

 specimen, but that G belongs to the same species as A is evident from the size of 

 the spines, the size and relations of the few tubular prolongations of the test that 

 can be recognised, the size of the ostia, and the characters of the polypides. 

 Specimen G is evidently a young colony of Cephalodiscus hodgsoni which has not 

 yet assumed the racemose appearance presented by the type specimen. The 

 polypides are crowded underneath a square inch or so of the calcified reticular 

 coenoecium of the polyzoon Retepora, in relation with which the colony has 

 established itself. They are supported upon a fairly flat floor of test secreted by 

 themselves, and the upper surface of the Retepora is also coated with a thin, flat 

 sheet of the same material. From around three-fourths of the edge there stand 

 out fifty or sixty spines, mostly simple and unforked, and radiating more or less 

 obliquely outwards from the centre of the whole structure. Some of these spines 

 are arranged in groups of four or five around the openings of ten or eleven 

 short tubes which lead out from the marginal parts of the flattened central 

 chamber. 



This specimen (G) is interesting because Harmer, in describing the species 

 C. siboyae, attaches some importance to the crowding of the polypides in a low and 

 wide " basal encrustation " with irregularly divided cavity, set upon the piece of 

 rock on which the colony was growing (10, p. 13). That Cephalodiscus sibogae is 

 a valid species I do not doubt, but this particular " habit " of the colony will 

 probably prove to be common, in the earlier stages of growth of the colony, to 

 most, if not to all species of Cephalodiscus, except those which, like C. levinseni 

 and C. nigrescens, have separate tubes for each polypide and its buds. 



Polypides. 



It is remarkable how densely crowded are the polypides in this species ; yet, 

 although they are in such close contact, the only organic continuity which exists 

 is that between the parents and their buds. If one side of the tubarium be removed 

 and the contents carefully isolated, the crowd of polypides can be unravelled into 

 a number of separate individuals, with their buds. The apparent connection is 

 due to the entanglement of the stalks of the buds of the different individuals. 



Judging from the least crowded parts of the colony it is evident that, as in 

 C. dodecalophus, each polypide is free to pass out through the ostia of the tubarium, 

 and to move about from end to end of the interior, except in so far as it may 

 be incommoded by the crowding of the other polypides. 



