690 R. KIRKPATEICK. 



Poi'to Santo is a little island about six miles long from east 

 to west, with a small islet, Cima,^ at its south-east corner, 

 and another, Ilheo de Baixo, at its south-west corner. Baixo, 

 which is a volcanic island, is extremely interesting- geolo- 

 gically, for it has beautifully preserved coral reefs sand- 

 wiched between layers of basalt. The Madeifans use the 

 coral for lin:e, and the English name of the island, viz. 

 Lime Island, is of some significance from the point of view of 

 a theory I am about to suggest in explanation of the peculiar 

 feature of Merlia, viz. that this little sponge may have 

 acquired its lime-forming character at the epoch when the 

 coral reefs of Baixo were being formed, that is to say, during 

 the Miocene period.^ 



There would probably have been a larger amount of carbo- 

 nate of lime in solution and of hne particles of that substance 

 in suspension in the sea-water in that area than at present. 



Possibly the metabolic cells of Merlia became fed up (if a 

 vulgar colloquialism may be permitted in the classic pages of 

 this journal) with calcareous matter, and either re-secreted it, 

 or became wholly calcified into lumps or flakes."^ Nature, always 

 selecting what is helpful for survival, ended in m.ikiiig use 

 ol this property to elevate the delicate little crust — so liable 

 to be overlaid by other oi-ganistns — and to form shelter-pits 

 for the soft tissues. Support and shelter are two very 

 important factors in sponge economy, as witness the boring 

 sponges, which have acquired the character of working theii- 

 way into shells and limestone. Merlia constructs its own 

 shelter and supports. Morphologicnlly the calcareous skeleton 

 is wholly on the basal surface of the sponge, and, indeed,, 

 ■outside of it. 



' Cima ahove. Baixo (cf. basis) low. The Portuguese refer to a looa- 

 tion to the east as cinia and one to the west as l)aixo, because their own 

 moiuatains are east and their coasts Avest. 



- It would be interesting to discover Merlia incrusting the shells, 

 corals, etc., so abundant in these reefs. It seems to nie very probable 

 that it will be found tliere. 



=* Compare Haeckel's theory of the origin of calcareous and siliceousy 

 ■sponges respectively on calcareous and siliceous oozes. 



