28 Coelenterata. 



cause rapid erosion of the limestone, so that the lagoon is becoming larger. 

 The land-rim is thus rapidly decreasing in size, and is being divided into 

 islands by fresh channels, which are being formed outwards from the lagoon. 

 As soon as the rock of the land-rim has been cut down below low water- 

 mark, it is colonised by corals and Lithothamnia, which protect it from further 

 erosion, while above low water-mark it is unprotected. The lagoon is barren, 

 live corals being limited to the regions of the channels. Near the largest 

 channel there is a definite, flourishing reef, containing a varied assortment of 

 corals, though Tubipora is unusually common. A fringing reef extends entirely 

 round the atoll (except at the channels), varies in width from 500 to 200 yards, 

 and consists of a coarse rubble of broken coral-fragments, all much encrusted 

 with Lithothamnia; there are few living corals. The fringing reef is not 

 a sign of the growth of the atoll seawards, but of the amount of its loss of 

 land due to erosion. Astove is an elevated atoll, the land-rim of which is 

 pierced by one shallow channel, leading to the very shallow lagoon. This 

 channel is of recent formation. Cosmoledo is also an elevated atoll (the elevation 

 can not have been less than 12 feet) and its land-rim, once almost perfect, has 

 been broken up by erosion. Assumption is a crescent-shaped reef, resting pro- 

 bably on a narrow summit, and has undergone elevation, for corals in position 

 of growth are abundantly present along the cliffs. Farquhar atoll has a very 

 imperfect land-rim, it consists of elevated coral-rocks, cut down below low tide- 

 mark, and now protected from further loss, indeed, it may be gaining owing 

 to the rigorous growth of Lithothamnia. All the evidence gained by a study 

 of these atolls points to a method of formation by elevation and it is suggested 

 that the bases of the atolls are volcanic wounds. -- See also Crawshay, Gravierf 1 - 3 ), 

 Nordgaard, Ritchie, Stephens and Wichmann. 



Howe points out that lime-secreting plants (Lithothamnion , etc.) play an 

 important and often a predominant part in reef-building, that such plants 

 appear to be much more generally distributed than corals, and flourish in greater 

 depths. 



Yakowlew explains the bending of the polyparium of the Hugos a as due to 

 its having been attached by the lateral wall near the lower end. The scar of 

 fixation (in Gyathopliyllum] does not involve the whole periphery near the lower 

 end of the coral, but only one side, whereas in Hexacorallia the scar is generally 

 at right angles to the axis. In default of a suitable base of fixation, the 

 Rugosa either retain the form of a bent cone, or assume a flattened condition 

 (e. g.j PalcBocyclus). This flattened secondary form, however, preserves its bi- 

 lateral symmetry; in Calceola, etc., this is clear, but in the more radially 

 symmetrical P. there is also bilateral symmetry, the original growing-point, 

 seen on the lower side of the corallum, being always excentric, the line of 

 greatest distance from it to the periphery corresponding to the main septum, 

 and, consequently, to the convex side of the polyparium of the bent forms. 

 The conical polyparium of the Rugosa, after growing to a certain size, forms 

 a series of conical calices, each arising from the distal end of the preceding. 

 In many cases, however, such a rejuvenescence does not take place, but the 

 coral continues to grow in a cylindrical form, in which the number of septa 

 remains unchanged, but the differences in size between the various septa dis- 

 appear, i. e., the bilateral symmetry is lost; this is not an ontogenetic repetition 

 of a phylogenetic transition of the Rugosa into the Hexacorallia. The characte- 

 ristics of the Rugosa were maintained throughout the whole palaeozoic period; 

 this supports the view that the Rugosa became extinct without emerging into 

 the Hexacorallia. The inter-septal chamber on each side of the main 



