SUB-GENUS PERONOPORA. 217 



The corallites spring from both sides of a median plane, which 

 divides the coralhim into two flat laminae, and which is some- 

 times marked by a thin calcareous membrane (probably not a 

 definite structure). The corallites are at first oblique and thin- 

 walled ; but they almost immediately bend outwards, and pro- 

 ceed straight to the surface, their walls becoming at the same 

 time moderately thickened. The larger and smaller corallites 

 of the colony exhibit a totally different form of tabulation. 

 The large tubes possess, in part or throughout their entire 

 length, peculiar incomplete tabulae, which form a series of 

 overlapping lenticular vesicles on one side of the visceral 

 chamber, and which may or may not be connected with the 

 opposite wall of the tube by a few straight and horizontal 

 plates. On the other hand, the interstitial corallites are fur- 

 nished with numerous, close - set, horizontal, and complete 

 tabulae. 



Obs. — In its general conformation and mode of growth this 

 species resembles normal examples of j\I. viannniilata, D'Orb., 

 and ]\I. vwlcsta, Nich. That is to say, it forms flattened, irregu- 

 larly undulating fronds, the size and thickness of which vary 

 with age, and which are composed of two strata of corallites, 

 the apertures of which are placed on the opposing flat faces of 

 the corallum, while they spring at their bases from the opposite 

 sides of a median plane. This plane, in thin sections taken at 

 right angles to the flat faces of the corallum, is marked by a 

 delicate line representing, apparently, the cut edge of a mesial 

 calcareous lamina (fig. 46, d). As, however, the corallum 

 shows no tendency to split along this line, and as neither of 

 the apposed strata of corallites can be stripped off separately, 

 it appears doubtful if this lamina Is a definite structure, and it 

 seems more probable that it is simply formed by the union of 

 the bases of the two sets of tubes. 



The surface - characters of the species vary considerably. 

 All alike show the large openings of the larger corallites sur- 

 rounded by numerous Intercalated small corallites, and all alike 

 show rounded or somewhat stellate spaces, which are occupied 



