30 MADREPORARIA. 



two genera.* Of Didyarcea Reuss, we only know the small thin branched forms found fossil 

 in the early tertiary strata of Java. These again may be true perforates, and might perhaps be 

 regarded as specialised branching forms of the Poritidae. But the thin walls on the sides of the 

 stems where the Poritidae usually have especially thickened reticular walls suggest an origin 

 from simpler colonies in which the perforate walls were typically very thin. This is the case in 

 Alvcopora, from which the other structural details of the genus might also perhaps be deduced. 



V. DIAGNOSIS OF THE GENUS. 



The Gonioporce are astrseiform colonies of perforate corals, that is, of corals whose thecse 

 are built up purely of septa united by synapticulte. The original flattened epitheca persists 

 not merely to protect the colony from the substratum, but as an important constituent of the 

 skeleton, being especially prominent at the free edges. The perforate septa typically number 

 twenty-four, and show bilateral symmetry in the septal formula, there being two primaries 

 as directives. On each side of the directive plane, the remaining septa fuse as follows : 

 two quartets, each composed of one secondary fused with one primary, the two tertiaries fusing 

 with the secondary, and one triplet composed of one secondary with its two fused tertiaries (see 

 the diagram, p. 21). The points where the fusions take place frequently rise as knobs and form 

 the pali which are thus typically six in number, viz. three on each side, two large for the 

 quartets, and one small ; sometimes the tips of the directives also form small pali, thus making 

 eight. The position of the directives is sometimes indicated in the middle of the calicle 

 by a tubercle which may be flattened in the directive plane. The original septal formula is, 

 however, frequently obscured and at times only traces of it can be recognised. The inner edges 

 of the septa frequently unite to form a columeUar tangle. Owing to the porosity of the 

 skeleton, and to the consequent intercommunication between the parts of the colony, the 

 skeleton of the lower lying portions is always much thicker and more solid than that of the 

 upper portions, according to the law which pertains throughout the whole coral system, that 

 the basal portions of the skeleton are invariably the thickest. 



VI. (a) DISTRIBUTION IN SPACE. 



The exact distribution so far as at present known shows that the genus is now confined 

 entirely to the coral reefs of the Indo-Pacific area. It is recorded from the Pacific not further 

 east than Samoa, although it is doubtful whether this can be its extreme eastern limit. It is 

 fairly abundant round the northern shores of Australia, through the Malay Archipelago into 

 the China Sea, across the Indian Ocean and in the Red Sea. 



* Miss Ogilvie (' Die Korallen der Strassburger Schichten,' Palaeont. Suppl. II. 1896-97 ; see 

 also Phil. Trans., clxxxvii. (1897) p. 211) has suggested an association between Adinacis and 

 Turhinaria, based chiefly upon sections ; the slightly exsert rims of the calicles, never found in the 

 Poritidae, is further morphological evidence in favour of this suggestion, while the fact that cretaceous 

 Turbinarians are known affords some justification, geologically, for associating them together. 



