CORALS FROM THE PERSIAN GULF. 1023 



Aspidosiphon ; the flat base of the normal specimen is never so 

 large as the caUce, and this shape is invariably in correlation with 

 alternating costte, thin septa, a deep fossa, and an ill-defined 

 columella. 



The characters of the costfe of the two types are very definite. 

 Fig. 12 on Plate LYIII. represents two adjacent costag of Type I, 

 low broad ridges, covered all over with fine uniform granules ; the 

 intercostal furrows are small and shallow. Fig. 13 on Plate LYIII. 

 represents two adjacent costte of Type II : on the right is the 

 type of costa which corresponds to septal cycles i., ii., and iii., 

 a prominent exsert ridge, on which the granulations tend 

 to become regular transverse bars ; on the left is a costa corres- 

 ponding to septal cycle iv., slightly less exsert, and beset with 

 very irregular coarse granulations ; the intercostal furrows are 

 deep. These marked differences, always in correlation with the 

 difl:erences of the calicular characters to which reference has 

 already been made, are too consistent to retain both forms in the 

 same species, True, there are many examples of Type I which 

 have an alternation of broad and narrow costse, but such coslfe are 

 always alike in being low and uniformly granular ; the alternation 

 of coarsely granular costse and much exsert nari'ow costse is never 

 discernible. In some, however, there is a tendency for the costpe 

 of Type I to become more exsert and more coai-sely gianular at 

 the calicular margin, but this generally occurs v/here the whole 

 growth of the coral has been distorted by being fixed to an 

 abnormally large shell, and it might be thought that this distortion 

 had influenced the form of growth. Indeed, it suggested the 

 possibility that the shell on which the coral fixes itself, and the 

 position which that shell assumes within the actual body of the 

 coral, might influence the mode of growth to such an extent 

 that not only the general shape but the character of the septa 

 and costpe might be controlled. This, however, is not the case. 

 There are well-defined examples of Types I and II both fixed 

 on exactly similar shells : in some the Aspidosiphon in corals of 

 both types is coiled horizontally forming a flat base, in others the 

 corals are fixed to shells which lie sometimes horizontally, 

 sometimes vertically within the coral zoophyte, and there are 

 always examples of both types harbouring Aspidosiphons in shells 

 which assume either position ; so that the sjiecies of shell or 

 the position it assumes exercises no influence on the manner of 

 growth of these two well-defined types. Such types vary about 

 two distinct modes, with a slight overlapping of the extremes of 

 variation ; and are not themselves the exti-emes of a single growth 

 mode. The relation of the species to each other is represented in 

 text-fig. 216. 



Of some 252 individuals in the South African collection, there 

 are 225 of Type I and 19 of Type II, and 8 which have characters 

 common to both types. The characters which separate the two types 

 cannot be measured and given numerical values ; consequently 

 in text-fig. 216 A the point about which the two growth-modes 



