1 7S Royal Society : — 



April f). l"<7i;. — Ur. J. Daltoii Hooker, C.H., I'rosiilcnt, in the 



Chair. 



"Oil the Structure of a Species of MiUepora occurring at Taliiti, 

 Society Ishinds." By H. X. ilosKi.EY. Naturalist to the 'Challenger ' 

 Expedition. 



In a paper treating mainly of the structure of Jleliopora cm-ulea, 

 communieated to the Royal Society in the autumn of 1875, some 

 account was given of results arrived at from the examination of 

 two species of MiUepora obtained at liermuda and at Z-imboangan, 

 Philippines ; and in that paper a summary of the literature con- 

 cerning the tabulate corals generally was given. The present 

 paper, to be considered to a certain extent a continuation of the 

 last, gives an account of the structure of a species of MiUepora 

 obtained at Tahiti, Society Island'^. The author commences by 

 expressing his obligations to his colleague Mr. J. Murray, who ob- 

 tained living specimens of the MiUepora and handed them over 

 to him with the zooids in the expanded coiidition for examinatioti, 

 and who further, having devoted some time to the study of the 

 coral, gave him valuable information with regard to several points 

 in its structure. 



No MiUepora appears to have been hitherto known to occur at 

 Tahiti. The name of the species of the one the structure of which 

 is described in the paper was not ascertained. It resembles M. 

 tuberculosa, as described by Milne-Edwards *, in outward form, 

 but differs from it in having the calicles of two kinds disposed 

 on the surface of the corallum in regular separate systems, in this 

 respect resembling more closely M. plirata, M. foliata, and M. 

 Ehrenhertjii as described by the same author. 



The coral was examined in the fresh condition, and also pre- 

 served in alcohol, chromic acid, and glycerine, and treated with 

 osmic acid. Hardened specimens were decalcified and examined 

 bv means of sections. The corallum is a spongy mass composed 

 of more or less contorted trabecul<e of calcareous matter, which 

 is disposed in a series of thin layers following the contours of 

 the surface, and representing successi\e additions by growth. 

 Within these layers ramifv a series of canals which give off 

 branches and subbranches, the whole ramifications being intimately 

 connected with one another, and with the calicular cavities, by a 

 network of smaller channels. The main canals are sometimes large 

 enough to be easily seen by the naked eye, and run for as great 

 a distance on the surface of the corallum as 1| inch. This system 

 of branching canals is held to be characteristic of the coralla 

 formed by the hydroid genus MiUejiora, distinguishing it from all 

 other coralla. The calicles are of two kinds, small and large. They 

 are disposed on the surface of the corallum in irregularly circular 

 systems. A large calicle occupies the centre of each system, and 

 is surrounded by a ring of smaller calicles, usually from five 



* Hist. >'at. fles Cura Ilia ires, pi. F^. fig*, l'^'. !''• 



