CORALLUM OF TURBINARIA. 361 
grown; though it by no means follows that heredity plays no 
part in determining the form of growth assumed by the corallum 
under any particular conditions, and it may well be that the 
tendency towards one type rather than another is inherited; this, 
however, can only be established by experiment. 
As might be expected, the largest and most perfect cups are 
those formed at depths below the tidal zone, in clear water, and 
where the growth of the corallum is unrestricted by neighbouring 
objects *. 
Above extreme low-water mark there is a greater tendency 
for the coral to lose its cup-shape, and to become irregular by 
_the folding and crumpling of its walls and by the adoption of 
an encrusting habit. Again, specimens are common on the 
reefs in which certain calicles have budded to form secondary, 
more or less independent, colonies: subsidiary cups may thus 
be formed within the parent cup, and some such individuals 
present a regularly “storied” appearance. In other cases the 
secondary colonies, instead of forming cups, take on an arbo- 
rescent growth like that of a Madrepora. This modification f, 
which I may term the “ madreporiform ” type, is a not uncommon 
one where the coral is growing at the bottom of a hole in the 
reef, and where growth in a vertical direction is of obvious 
advantage to the colony. 
When a Zurbinaria grows upon a shelf or ledge of rock it 
generally loses its cup-form; the side turned away from the 
free edge of the shelf ceases to grow, and the corallum thus 
becomes a more or less flattened, expanded plate§ overhanging 
the ledge. 
When, during growth, the lower surface of the cup comes into 
contact with the substratum, irregular root-like outgrowths will 
* At the time of publication of the British Museum Catalogue the largest 
cup in that collection was stated (with a certain amount of pride) to measure: 
as much as sixteen inches in diameter; such a specimen is, however, in reality 
quite a small one compared with the giants occurring on the shelling grounds 
in Torres Straits. 
+ These daughter cups, the result of proliferation of individual polyps, must 
not be confounded with the cup-shaped folds of the wall of the parent cup, 
which are of much more common occurrence. 
+ A very good example of this type of growth is figured by Ortmann as 
Ee, acai Foal. Jahrb., Syst. vol. iii. pl. vi. fig. 4. 
§ The specimens of 7. reniformis and T. foliosa figured by Berned 
(Catalogue, pls. xvii. & xviii.) probably owe their form to this cause. 
