42 THE VOYAGE OF H.M.S. CHALLENGER. 
very little value can be given to the character that the septa of the Rugosa are generally 
of two sizes. 
As regards the fourth point, it must be conceded that tabulee are more characteristic 
of the Rugosa as a whole than they are of the Astrzids, though exceptions occur on both 
sides ; for while there are Rugosa which are destitute of tabulee, there are also Astreeids 
which possess them. It is moreover a very doubtful point as to what value should be 
placed on the presence or absence of these structures, considering the high degree of 
development to which they attain in other and widely distinct Aporose Madreporaria. 
As regards the fifth and last point, it must be stated that the absence of a true 
coenenchyma in the compound coralla, and the increase by calicular gemmation are as 
sharply characteristic of some of the most highly developed of the Astraeids (as for 
instance, the forms of the genus Prionastrea) as of the Rugosa. 
Thus, as the result of the foregoing considerations, there is not a single characteristic 
of the old group Rugosa which will essentially separate its forms from the more typical 
Astreeids; and a direct expression is given to this fact by placing the families of the 
old Rugosa (except the family Cyathaxonidse, which has been placed under the sub- 
section Turbinolida) with the family Astreeidee, under the subsection Astreeida. 
An extremely interesting comparison of a typical recent Astreeid with a Cyathophyllid 
may be made by selecting specimens of Prionastrea favosa (= Madrepora favosa, Ell. 
and Sol.) and the compound form of Cyathophyllum helianthoides, Goldf. In the former 
the calicles are large, being about 30 mm. in diameter, and deep, about 10 to 15mm; the 
walls are simple, and excessively thin above, and im transverse section are seen to 
be very shghtly developed and vesiculate; the septa are numerous, often about 48, and 
subequal, projecting nearly evenly to the centre, where their innermost teeth give rise to 
a false columella; and the dissepiments are very abundant, oblique and vesiculate. In 
Cyathophyllum helianthoides the foregoing characters are almost identical, but the 
dissepiments are placed higher, leaving but a small extent of septa free exteriorly, 
and at the centre give place or give rise to slightly developed tabule. 
This comparison, indeed, brings to light not only the essential relation between the 
two types, and the close affinities of the families to which they belong, but points to the 
most constant difference which will serve to distinguish the Astreide and the Cyatho- 
phyllidze, while the generally prevailing hexameral arrangement of the septa in the former 
will serve stil] further to limit them—characters in both cases, it must be confessed, of 
doubtful value. 
The relation of the form Moseleya latistellata to the Astreeidz, as exemplified in 
Prionastrea favosa, Ell. and Sol., is equally striking and suggestive. In Moseleya 
latistellata the calicles are as large as or larger than those of Prionastrea favosa; the 
septa are much more numerous, markedly of unequal sizes, often continuous from calicle 
to calicle, and give no indication of hexameral arrangement, while in a simple young form, 
