SUB- GENUS PERONOPORA. 225 



formly of a pronounced and markedly elevated character than 

 is the case In all specimens of the latter. The normal mode of 

 growth in M. molcsta is certainly that of an expanded frond, 

 composed of two strata of corallites, which diverge in opposite 

 directions from an imaginary mesial plane to open on the two 

 flat surfaces of the expansion (PI. VI. fig. 2). As in the case 

 of M. mammulata, the central line of the corallum is not 

 marked by any even imperfectly developed calcareous lamina, 

 such as exists in M. frondosa, D'Orb., and M. pavonia, D'Orb. 

 Though usually frondescent, I possess one large massive and 

 lobate specimen (four inches in length, three inches in width, 

 and two and a half inches in height) which is proved by micro- 

 scopic examination to be unquestionably identical with the more 

 ordinary examples of the species. 



As regards its internal structure, tangential sections (PI. VI. 

 figs, lb, 2c) show that the large corallites of the colony are 

 polygonal and thin-walled, the lines of demarcation between 

 adjoining tubes not being persistent, and the walls (in all the 

 slides I have examined) exhibiting a peculiar granular appear- 

 ance. The visceral chamber of each of the large corallites is 

 also crossed by a delicate lamina, which is deeply excavated 

 on one side in a crescentic or triangular manner, and the 

 presence of which is due to the intersection of the peculiar 

 vesicular tabulae on one side of the tube. Interspersed among 

 the large corallites, but never In large numbers, are smaller 

 angular interstitial corallites, which are recognisable not only 

 by their dimensions, but also by wanting the peculiar excentrl- 

 cally perforated tabulae of the large tubes. 



Vertical sections (PI. VI. fig. 2d) show that the walls of all 

 the corallites are thin, and that there is a conspicuous differ- 

 ence In the tabulation of the large and small tubes respectively. 

 The small corallites are uniformly provided with numerous, 

 close-set, complete, and horizontal tabulae. On the other hand, 

 the large corallites possess the same singular incomplete 

 tabulae as are seen In such forms as M. Selzuynii, NIch., M. 

 Cinciiimitiensis, James, M. Neivberryi, NIch., &c. That Is to 



