364 MR. S. PACE ON THE 
cups bearing polyps only upon their outer faces*; in such an 
example any mud settling within the cup will of course not 
affect the polyps. The derivation of the above-mentioned types 
from the primitive cup or disc is illustrated very diagrammatically 
in figs. 11 to 14 (p. 363). 
What Bernard has termed the “tabulate” type of growth is 
certainly, as he suggests, expressive of periodicity in the growth 
of the colony; and this periodicity appears to be often de- 
pendent upon the monsoons. The “set’’ of a current over a 
reef, and consequently also the “lay” of the silt, is in many 
places markedly different at these seasons; and, with every 
change in the direction of the drift, those parts of a coral which 
have been overwhelmed and killed by silt will tend to become 
again exposed, and may then take on a fresh Jease of life, while 
the opposite face of the colony may in turn be buried until 
another change takes place in the set of the current. It is 
always possible to find some evidence of this periodicity on any 
large block of Porites, Turbinaria, &e., as it occurs on a reef; 
and, though of course many other factors besides the monsoons 
are concerned in effecting those changes which are continually 
taking place in the set of a marine current, yet such extremely 
regular alternations, as are expressed by the typically “ tabu- 
late” type of Turbinarian growth, can only be due to their 
succession. 
The “clomerate’’ type, in which the corallum becomes 
enormously thickened to form the large hemispherical masses + 
so common on many reefs, would appear to be an adaptation 
mainly to withstand the battering of the surf and the rush of 
the tide over the reef. In many localities the strength of the 
current is so great that a corallum of any other form would 
most certainly be swept away; and on very exposed situations 
massive forms of Turbinaria, Porites, and such-like are the only 
corals met with, 
* A specimen referred to 7. magna, Bern., in the Saville Kent collection in 
the British Museum from Shark’s Bay, Western Australia, is a very perfect 
example of this type of growth, which is also to be seen in some parts of the 
specimens figured in the British Museum Catalogue, pls. xil., xiil., & xiv. 
|} Examples of these are contained in the Saville-Kent collection in the 
British Museum, and many such may ke recognized in the beautiful collotype 
plates of coral-reefs which illustrate Saville Kent's ‘Great Barrier-Reef.’ 
