12 GENEEAL EEMAEKS. 



in the radial corallites the directive septa are most frequently better developed, either stouter 

 or broader, than the other primaries. In certain groups of species, however, the outer 

 directive septum is more important than the inner ; and in case only one septum is present 

 it is invariably the outer directive. Apparently the number, but more particularly the 

 relative importance, of the septa form characters of value for the classification of species. In 

 several of the subdivisions now proposed the species were first associated together on general 

 grounds, and without any regard to the condition of the septa ; and yet, in almost every case, 

 the relative importance of the septa will be found nearly constant — e. g., Lepidocyathus and 

 Trachylopora. In certain species there appear to be two types of radial corallites. In the 

 usual more or less prominent one the septa are naturally least developed in the young buds 

 near the apex, and there is a gradual increase in importance in older corallites until the 

 normal condition is attained. It may be that the condition of the septa is subject to little 

 variation whether the corallite be prominent or immersed, situated near the apex or near the 

 base of the corallum. In certain species, however, some of the immersed corallites of the 

 upper surface of the corallum have quite a different development of the septa to the 

 prominent ones. Sometimes the septa in these immersed corallites are more numerous 

 or better developed — a condition which might be attributed to agej but that this is not 

 always the case is clear from the fact that such corallites often contain more septa than the 

 axial corallites, or, on the other hand, may show an almost complete absence of septa. 

 Similarly, prominent corallites on the under surface of prostrate or vasiform specimens may 

 show a condition of the septa quite distinct from that characteristic of the upper surface. 



Whether in the case of the axial corallites the relative importance of the directive septa 

 is usually associated with a variation in the size of the tentacles situated over them is not 

 known. In Dana's figures of the axial polyps of M. cribripora and M. aspera the tentacles 

 are shown of equal length. In Agassiz's ' Florida Reefs ' the polyps of M. cervicornis and 

 M. prolifera are figured, and in both cases the tentacles are drawn alternately large and 

 small, corresponding evidently to the broader primary and narrower secondary septa. This 

 arrangement is in accordance with Lacaze-Duthiers's figures of the embryo of Astroides. 

 Mr. Saville-Kent informs me that in his experience of the Torres-Straits and Barrier-Reef 

 species the tentacles of the axial polyps are always of equal length, even in species in which 

 this does not hold good for the radial polyps. So far as I am aware, Dana does not anywhere 

 state the usual condition of the tentacles of the radial polyps in Madrepora in the living 

 specimens which came under his notice. There seems reason to suppose that they were 

 generally of equal size, from the fact that he figures them as such without comment in the 

 case of M. aspera. With regard to M. prostrata he calls special attention to the fact that 

 one of the tentacles is much longer than the others, and that this is situated over the outer 

 directive septum, which is itself broader and stouter than the remaining septa. That this 

 arrangement was not usual in his experience will be rendered clear from the following 

 quotation : — " Among the species of Madrepora there are some in which one of the tentacles 

 of the polyps is long and flexible. This was observed in a cespitose species [M. prostrata) 

 having thin labellate calicles ; but whether it belongs or not to all the horizontally-growing 



