Professor Herdman on the Structure of Sarcodictyon. 31 



die Darwin'sche Senkungstheorie." In this interesting paper 

 the author emphasises his dissent from the theory of subsi- 

 dence, and refers to the confirmation of his original suggestion 

 by the deep-sea explorations of recent years. In the ap- 

 pendix he sums up his conclusions, asserting that the as- 

 sumption of extensive submergence in the coral seas rests 

 on supposition, and not on exact observations, the supposed 

 vast thickness of modern coral-reefs being illusory and un- 

 supported by any measurements ; that the association of all 

 the kinds of coral-reef within the same limited area is 

 inexplicable on Darwin's theory ; that in no geological for- 

 mation, from palaeozoic up to recent time, are there any 

 coral-reefs approaching the thickness asserted to characterise 

 living reefs, their average depth being much less than 300 

 metres; that coral-reefs may be simply and naturally ex- 

 plained as the crowning of submarine banks which have 

 been sufficiently heightened by the accumulation of the 

 remains of plants and animals; and that the form of the 

 reefs, especially of the atolls, depends primarily on the shape 

 of the bottom and the food-supply, and is thus more simply 

 and naturally accounted for than by subsidence.] 



I. On the Structure of Sarcodictyon. By W. A. Herdman, 

 Esq., D.Sc, F.L.S., Professor of Natural History in 

 University College, Liverpool. [Plates I.-IIL] 



(Read 19th December 1883.) 



I. Introductory. 



While dredging in Lamlash Bay, Arran, during the sum- 

 mer of 1880, I was fortunate enough to obtain several good 

 living specimens of Sarcodictyon catenata (Forbes). They 

 were all small colonies of the usual red colour, and attached 

 to pieces of stone and dead sheUs. As I was occupied with 

 another group of animals at the time I contented myself with 

 identifying the species, and then preserved them in alcohol 

 for future use. 



Last September (1883), while dredging in the neighbour- 



