﻿24 MADEEPOEARIA. 



6. Montipora tenuissima. (PL I. fig. 5 ; PI. XXXI. fig. 6.) 



Description. — Corallum very thin, from 1 to 1"5 mm. tliick, less than 1 mm. at the 

 growing edge, irregularly horizontal, expanding over former growths, edges extending freely 

 but supported by a well developed concentrically wrinkled epitheca which is conterminous 

 with the growing edge. The epitheca is secondarily strengthened by a layer of dense matter 

 deposited on its upper surface by the coenenchyma. New growths arch here and there freely 

 over the old. 



The polyp cavities, which are irregularly distributed from 1 to 3 mm. apart, are minute, 

 ■ 75 mm. across, star-like, and somewhat conspicuous on the older and thicker portion of the 

 corallum, but very much smaller on the thin fragile expanding edges. Owing to the 

 extreme thinness of the stock the coenenchyma is slightly raised round the polyp cavities, 

 here and there into prominent and hemispherical mounds. Towards the edges the slightly 

 raised apertures slope outwards. The six primary septa are prominent and exsert, one or 

 two dii'ectives here and there still more prominent than the rest. Secondary septa not deve- 

 loped. The margin of the calicles quite irregular, the interseptal loculi running out into the 

 open spaces of the coenenchyma. 



In section, the typical horizontal streaming layer is recognisable. It consists of a wide- 

 meshed thread- Hive reticulum, being, in thin portions, some three meshes altogether in tliickness. 

 This reticulum forms a dense layer immediately in contact with the epitheca, while dorsally 

 it gives rise in the older thicker portions of the stock to a thin layer of upright threads. 

 These are thin and delicate, expanding, however, at intervals into thick nodes which fuse 

 with those of the adjacent threads. The coenenchyma on the surface is finely and evenly 

 granular, the granules being very minute in the expanding portions of the stock where 

 only the points of the streaming reticulum rise to the surface, but somewhat coarser in older 

 encrusting specimens where the thickening layer is developed : in this latter case, they are 

 the irregular knob-like or slightly branching expansions of the tips of the vertical elements 

 of this layer. 



There are three specimens united together under this heading. One is an irregularly 

 oval specimen 4 to 6 cm. across (PI. I.). It is encrusting a dead previous growth, but has 

 one edge freely expanding. There is a marked contrast between the caKcles on the thicker 

 encrusting portion and those on the freely expanding portion. In the former situation the 

 calicles are raised and conspicuous, in the latter they are hardly visible. 



The other two specimens are fragments from the free edges of other stocks, and are accord- 

 ingly very thin and fragile, and with inconspicuous calicles. The tendency of the coenenchyma 

 to form slight eminences round the polyp cavities in the extremely thin reticulum is, however, 

 evident. There is no thickening layer in these fragments, and the surface is smooth and 

 velvety. Both specimens have Balanids, and the larger one recumbent worm tubes as well, 

 ■on their upper faces ; over these the coenenchyma sooner or later spreads. The smaller of 



