22 MADEEPORARIA. 



stomodseum, and is situated between a pair of directive mesenteries. This condition is, 

 however, not characteristic of the genus {cf. p. 10). 



Further information regarding the structure of the polyps and the relation of the soft 

 tissues to the skeleton will be found on p. 14. 



With the exception of the peculiar mode of colony-formation, first clearly recognized by 

 Ehrenberg, none of the characters hitherto included in the definition of the genus are 

 absolutely constant throughout or confined to it. 



Synopsis of the Subgenera. 



Division I. — Madreporae with cylindrical axial corallites, which project to a greater or less extent 

 at the apex of each division of the coralluin ; wall usually very porous, margin plane, 

 exterior more or less distinctly striate or rugose. 



1. Corallam usually arborescent. Axial corallites 2-5 mm. diameter or 



over, with relatively thin wall and 12 septa, the primaries being subequal. 



Radial corallites usually with a distinct star of 12 septa Eumadbepoba, p. 23. 



2. Corallum more or less complanate. The second cycle of septa is absent or 



imperfectly developed even in axial corallites. The majority of the 

 radial corallites are usually immersed or subimmersed ; the more promi- 

 nent ones have the wall acuminate ODONTOCTATirtrs, p. 66. 



3. Corallum usually cespitose or corymbose. Branchlets more or less 



distinctly spiciform. A second cycle of septa occurs in axial corallites. 

 Radial corallites wita not more than the inner third of the wall 



undeveloped Poltstachts, p. 73. 



' 4. Corallum usually corymbose or prostrate. Radial corallites evenly dis- 

 tributed, scale-like, very spreading, giving a catkin-like appearance to 

 the branchlets. The outer directive septum of radial corallites is usually 

 better developed than the others Lbpidoctathtjs, p. 115. 



Division II. — Madreporw in which the axial corallites are stout, thick-waUed, often hemispherical, 

 not exsert at the margin, or only slightly so ; wall dense, rarely striate. 



1. Corallum consisting of massive plates, the upper border of which is 



occupied by numerous axial corallites, which are slightly exsert and 



have a rounded margin Isopoea, p. 131. 



2. Corallum cespitose, corymbose, or bushy. Branchlets blunt at the apex. 



Axial corallites relatively broad, hemispherical, or slightly exsert at the 



margin Tylopora, p. 135. 



Division III. — Axial coraUites relatively slender, conical or cyhndrical, in the latter case of con- 

 siderable length ; wall always dense. Many of the radial coraUites resemble the axial ones in 

 form. 



1. Corallum differing chiefly from that of Tylopora in having more slender 



apices to the branchlets and relatively small conical axial corallites .... Conoctathits, p. 160. 



2. Axial corallites cyhndrical, the prominent portion not usually longer than 



broad ; wall thick and dense, margin rounded. A second cycle of septa 



is present RHABDocriTHiTs, p. 173. 



