REPORT ON THE REEF-CORALS. 41 
5. The compound coralla possess no true ccenenchyma, and one of their commonest 
modes of increase is by means of calicular gemmation. 
As regards the first point, it must be remembered that modern investigation gives a 
constantly declining value to any essential difference between a tetrameral and hexameral 
arrangement of the septa. In many Astreids, not only is it an impossibility in the 
adult forms to prove the predominance of six septa, but also to derive the number present 
from such a type; for the total number present of those that are well developed is often 
not a multiple of six. And while in such a form as Stawria we are confronted with a 
distinctly marked cruciate arrangement, it is necessary to bear in mind that the 
transitional forms between the Stauride and the Cyathophyllide shew a decided decrease 
in the distinctness of such an arrangement, until in many species of typical Cyatho- 
phyllidee, as in species of Cyathophyllum, Lithostrotion, Acervularia, &c.,we arrive at a 
condition in which the septa are simply radially arranged without any indication whatever 
of a tetrameral type. 
As regards the second point, the presence or absence of a fossula is essentially a 
matter of but slight importance, and at most is recognised to be but of generic value, the 
genera which possess and those which do not possess a fossula being grouped together in 
the same tribe or subfamily, while to take a more especial case, as in the genus Cyatho- 
phyllum, in which a small septal fossula is often present, species which possess and those 
which do not possess this fossula are grouped under the same genus, the character in 
this case being recognised as not even possessing generic value. 
As regards the third point, it must be claimed that the presence of but two sizes of 
septa is as characteristic of a large number of the most typical Astreeids (species of 
Orbicella, Prionastrxa, &c.), as it is of the Rugose Coral. On the other hand, in the 
most typical of the Rugosa, in which the tetrameral arrangement is most visible, as for 
instance in Stawria, the septa are markedly of very different sizes according to the cycle 
to which they belong; a condition seen most clearly in a transverse section of the 
corallum, not only in Stawria but in many species of the Cyathophyllide, where by the 
union of the smaller septa with the larger, a clear idea of their inequality can be gained 
by the extremely variable position of their point of coalescence and its distance from the 
centre of the calicle.’ In Strombodes (asin Strombodes murchisoni) the extremely variable 
size of the septa is a most marked character; in species of Hndophyllum the variability 
among themselves of the smaller and larger septa is distinctly seen; while in many 
species of Cyathophyllum (Cyathophyllum stutchburyi, Cyathophyllum regium, Cyatho- 
phyllum rugosum, Cyathophyllum marmini, &e.) where roughly it may be stated that the 
septa are alternately equal, a transverse section sohws that there is a variability in the 
exact length not only of the shorter septa, but also of the larger which approach the 
centre, for while certain of them do actually reach there, many as certainly stop short of 
it. It seems to me that if transverse sections of typical Astreeids be taken for comparison, 
(ZOOL. CHALL, EXP.—PART XLVI.—1886.) Zz 6 
