GENERAL EEMARKS. 9 



surface of the cor'allum may be generally taken as a guide to the density within. In some 

 species, however, the thin wall of young corallites is fenestrated— that is to say, it consists of 

 longitudinal thicker bands connected together by a delicate and perforate lamina ; the thicker 

 bands may be considered costse, as they project beyond the intervening parts. In later 

 development the wall increases in thickness, and the costae become spinose ; still later, the 

 costfE become lost altogether, the surface-layer gradually becomes denser and is simply 

 clothed with spinules. In such species the costulate and simply echinulate conditions coexist 

 in different parts of the same colony, and the denser surface of the older corallites does not 

 correspond Avith the primitive density of the earlier-formed layers of the wall. As growth 

 proceeds, however, a further deposition of carbonate of lime takes place beneath the surface of 

 the corallum, so that a branch which is quite porous near its apex may become nearly solid 

 towards the base. 



Before leaving this subject, it appears desirable to discuss shortly the application of the 

 term costse in the case of Madrepora. The porous corallite-wall is essentially composed of 

 synapticulse, and is therefore not a thcca, as it differs both in structure and origin. In 

 thecate Madreporaria, and, indeed, in non-thecate types also, it is usual to apply the term 

 "costse-" to the longitudinal ridges which mark the outer surface of the wall. In the 

 majority of cases the costse correspond in number and position with the septa, and are usually 

 regarded as the distal extremities of septa which pass beyond the thecate wall. If G. v. Koch's 

 theory of the origin of a theca is correct, and considerable evidence has already been collected 

 in its favour, the costse should morphologically be considered as the distal parts of septa. In 

 the genus Madrepora the so-called costse undoubtedly do not come under this category. 

 They bear no regular relation to the septa either in number or position, and in point of 

 time appear before the septa and are also present on the walls of corallites in which never 

 more than two septa occur. Under these circumstances it does not appear desirable to 

 continue the use of one term for two perfectly distinct structures, and in the systematic 

 descriptions I have endeavoured to indicate the condition of the wall by the use of such terms 

 as fenestrate, striate, striato-echinulate, rugose, &c. 



It is often stated as characteristic of Madrepora that the directive septa (c/. p. 11) are 

 more prominent than the other primary septa ; but this is by no means always the case, nor 

 is that condition confined to the genus. In Madrepora one frequently finds that in radial 

 corallites the outer directive septum is broad and the other five primaries narrow and equal. 

 In other species the primary septa of both axial and radial corallites are of equal breadth, in 

 which case the bilateral arrangement of parts in the polyp is not indicated by the relative 

 importance of the directive septa. Again, in some specimens the six primary septa of the 

 axial corallites may all meet together in the axial fossa and fuse together so as to form a false 

 columella ; in other specimens only the directive septa become confluent. In Anacropora and 

 Montipora the relative importance of the primary septa is subject to similar variations to 

 those which occur in Madrepora. In Anacropora the corallites are prominent, and the 

 branches resemble those of Madrepora so closely that it is not until the absence of an axial 

 corallite is observed that the generic distinction is realized. In this genus the directive septa 



c 



