40) THE VOYAGE OF H.M.S. CHALLENGER. 
one hand, however, is clearly marked, since in many genera of the latter the condition of 
the septa, especially at their extreme peripheral portion, is closely similar to that which 
is normal throughout the whole internal structure of the Cystiphyllide ; while on the 
other hand doubtful representatives of the family present distinctly Aleyonarian features 
in their opercular characters. 
The Stauridze, among which is included the genus Holocystis, which has been placed 
by Professor Duncan in the Astraeidee, in their typical form differ from the more typical 
of the Astreeidee, as here limited, simply in the distinctly tetrameral arrangement of their 
septa. Very little value can be given to the absence of a columella or of coste in 
Stauria, for not only are these structures well developed in the genus Ho/ocystis, but 
they are among the characters of Astreeida some of the most variable in development. 
A variability in the distinctness of the tetrameral arrangement of the septa, even in 
the Stauridee, however, is clearly indicated in Metriophyllum, where, instead of the 
distinct and strong Maltese cross characteristic of Stawiia, are found the four bundles of 
septa which present a transitional character between this family and the Cyathophyllide, 
a transitional character originally pointed out by Milne-Edwards and Haime (Cor., iii. 
p. 328). 
It is difficult to draw any clear line of demarcation between the families Astreeidee and 
Cyathophyllidee, the more especially that certain genera of the one are clearly related to 
genera of the other possessing nearly identical characteristics. The usually accepted 
grouping of the two families seems to be based more essentially on their distribution in 
time than on any constant structural characteristics of real classificatory value. With 
a view more perfectly to grasp the essential characteristics tending on the one hand 
to relate and on the other to separate them, it will be instructive to examine, if 
only briefly, the main features which are supposed so to distinguish the forms 
commonly known as the Rugosa as to entitle them to the rank of a distinct order or 
suborder. 
While confessedly, in its most essential respects, the corallum of a typical Rugose 
Coral is quite identical with that of a typical Aporose Madreporian, it has been contended 
that the following more striking points of difference in the Rugose Coral are sutlicient to 
give it ordinal or subordimal rank. 
1. The septa appear to be primitively developed in four systems instead of six. 
2. The septa are rendered more or less irregular in their arrangement by the 
presence of a curious vacant space (sometimes three or four), which is known 
as the fossula, and which appears to take the place of one of the primitive 
four septa. 
3. When the septa are well developed, they generally present themselves in the 
adult as of two sizes only, a larger and a smaller. 
4. Tabulze are usually present, in conjunction with the septa. 
