74 Dr. G. J. Hinde on the Qenus Hindia. 



The otlier reason alleged by Prof. Duncan for his belief 

 in the original calcareous nature of Hindia is that the calcite 

 of which the spicular structure of the New Brunswick 

 examples at present consists is not in distinct crystals, and 

 cleavage-planes are rare, and the mineralization resembles 

 that of fossils which were originally of carbonate of lime. But 

 though this calcite is not in crystals, a very slight amount of 

 observation will show that it cannot be regarded as the 

 original mineral of the sponge-skeleton, since it is filled with 

 foreign dark grains and other particles of a similar nature to 

 those present in the matrix of the rock in which the sponge 

 has been imbedded. Its character shows that it has been 

 derived from the finer sediments of the surrounding rock, 

 which have found their way into the empty moulds left by 

 the dissolution and removal of the original siliceous spicules. 

 In fact, if we suppose the minute cavities in the silicified 

 Tennessee examples to be filled with fine calcareous sediment, 

 we should have structures produced like those of the New 

 Brunswick specimens. Under some conditions, instead of this 

 dusky non-crystalline material a true crystalline calcite has 

 filled up tlie cavities, as in the case of specimens from 

 Schoharie. 



The various mineral conditions under which Hindia occurs 

 are only such as may be found in fossil sponges which even 

 Prof. Duncan would not hesitate to accept as of siliceous origin, 

 such as, for example, the contemporary genera Astylospongia 

 and Aulocopium. In these sponges, as well as in Hindia^ 

 the original spicular structure may be either as empty casts 

 in a siliceous or calcareous matrix, or the casts may be infilled 

 either with granular sedimentary calcite or with crystalline 

 calcite, or with iron pyrites and peroxide of iron. 



But I have lately succeeded in obtaining further evidence 

 of the originally siliceous nature of Hindia by the discovery 

 of a portion of a specimen in which the spicules are actually 

 siliceous, and by the action of acid they can be isolated 

 from the matrix and obtained separately. In this condition 

 their surfaces are pitted and the expanded ends of the rays 

 eroded in precisely the same manner as the siliceous spicules 

 of many Cretaceous sponges. 



Possibly it may be urged that these siliceous spicules are 

 merely replacements of calcite by silica ; but, on the other 

 hand, in their form and character, and in their mode of union 

 with each other to form the skeleton, they so distinctly 

 resemble the siliceous spicules of both recent and fossil 

 lithistid sponges, that the conclusion is inevitable that they 

 must belong to the same group. 



