16 MADEEPORAEIA. 



to the branching forms. The union of all these under one species, " P. clavaria," by Dr. 

 Gregory under one species, " P. Porites," with three formce, clavaria, furcata and divaricata, 

 by Dr. Vaughan, was in reality a confession of despair at ever being able to find any growth 

 principles according to which they might be classified Dr. Vaughan's formm suggest a rough 

 grouping according as the branches are thick and clavate, or thinner and more openly forked, 

 or again very thin and divaricate. Tliis division, as far as it goes, is certainly a step in the 

 right direction, although it is necessary to caution the student against any confusion which 

 might be caused by the use of these names. For, as already stated (p. 5), Lamarck's two 

 names, clavaria and furcata were not meant to imply any such contrast between his type 

 specimens as is clearly meant by the current use of the terms. It is, however, only a very 

 small step, for it is far too crude, and there is no wonder if upon such a rough-and-ready 

 method of division, large collections show almost unlimited intergradings. 



Fortunately our study of the forms in the National Museum, as well as in the Natural 

 History Museum in Paris, compared also with the excellent figures published, among others, 

 by Dr. Eathbun and by Dr. Vaughan, shows that a law of repetition of growth prevails among 

 the branching, as well as among the encrusting and massive forms. 



Referring to Divisions 3 and 4, the one rises from a small disk, which acts as a basal 

 plate to a cylindrical stem, while the other rises at once, and swells as it rises. Both of them 

 attain a certain height, and then divide, forking at various definite angles, and showing different 

 kinds of division. Now my experience is that whatever the angle and the kind of forking in the 

 first instance, there is a tendency for the same to continue, and at the same intervals apart, 

 throughout all the subsequent growth of the stock. This repetition is perhaps more difficult 

 to establish than in the case of the explanate massive forms, because not only are growing 

 branches more exposed to external influences, but' mutual interference is certain sooner 

 or later to necessitate modifications. The principle can however be traced in most forms, 

 while in young stages, say for the first two or three successive forkings, it may be quite 

 conspicuous. One very startling case, though of only two successive forkings, with faint 

 indications of the third, may be cited in P. West Indies x. 14 (PI- XIII. fig. 3). In this case 

 each prong of the fork is twisted a little, not only on itself, but also out of the plane within 

 which the forking, in order to be rectangularly symmetrical, should have taken place. This 

 double twist occurs in the first forking, is repeated again almost exactly in the second, and is 

 already quite traceable in the beginnings of the third. This case of such exact repetition is 

 very remarkable, and, owing doubtless to the scarcity of young branching forms showing the 

 initial stages of branching, is not easy to confirm by the production of examples equally striking. 

 The majority of our specimens are fragments of old stocks, often growing upon overturned 

 earlier growths, and such like. But when we find a complicated twist, which might easily 

 be attributed to an accident if seen on one prong of a fork alone, not only produced by 

 both prongs, but by both prongs for two and even apparently three successive forkings, its 

 significance, as indicating some law of repetition in growth of the successive parts, cannot be 

 overlooked. 



