SUB-GENUS HETEROTRYPA. 107 



form corallites " has notably diminished, showing that these 

 latter do not extend to great depths below the surface. 



I have not succeeded in definitely recognising the " monti- 

 cules " in tangential sections, which may be taken as showing 

 that, on the whole, they do not differ materially in structure 

 from the bulk of the corallum. Sometimes, certainly, the 

 tubes of which they are composed look a little larger than the 

 average ; but this may very possibly be an illusory appearance, 

 and may be due to their greater nearness to the eye of the 

 observer. In other cases, beyond a doubt, many of the tubes 

 which are present in the monticules are below the average 

 size. 



Vertical sections, taken at right angles to the plane of the 

 frond (PI. VI. fig. i^), show, as before rem.arked, that the 

 corallites of the opposed sides of the corallum do not spring 

 from a definite mesial lamina, but simply diverge from one 

 another at their bases. At first slightly oblique and thin- 

 walled, they almost immediately bend outwards, their walls 

 at the same time being moderately thickened by a deposit 

 of light-coloured sclerenchyma, and they are then continued 

 nearly at right angles to the surface on which they ultimately 

 open. Sections of this kind show that all the tabulae are com- 

 plete and approximately horizontal ; but there is a marked 

 difference in the tabulation of the large and small corallites 

 respectively, the tabulse of the former being few and remote, 

 while in the latter they are numerous and close set. 



There is no doubt that the normal form of the corallum in 

 this species is that of a flattened undulating frond ; but I 

 possess one large and massive specimen, some three inches 

 in height by four in width, and two inches in thickness, which 

 has all the superficial characters of this form, and most of 

 its microscopic features. It differs, in fact, from the normal 

 examples of this species in no other peculiarity except that 

 the number of the small interstitial corallites is markedly 

 reduced, while its monticules are, perhaps, rather more pro- 

 nounced than is common in this type. So far, however, as I 



