178 Dr. H. Rauff o;i the Genus Ilindia, Dune. 



elements described. Even under the lens we also see on 

 radial fractures that these interstices everywhere form the 

 triangle corresponding to the tetracladine spicules ; but here 

 and there the thin sections also show the scanty rudiments of 

 the above-mentioned siliceous husks. 



In the examples from New Brunswick, on the other hand, 

 the skeleton itself is completely calcified, while the infilling 

 of the mesh-spaces is siliceous. At the first glance it may 

 perhaps appear most natural to deduce from this that the 

 skeleton was originally calcareous ; more careful consideration, 

 however, and a whole series of phenomena which occur in 

 these as in other silicified sponges prove that this lime is of 

 secondary nature, a secondary infiUing-mass of the originally 

 siliceous trabeculte which have been excavated during fossili- 

 zation. Almost all the completely silicified sponges which 

 have hitherto been investigated (Astylospongidaj, Aulocopia, 

 &c.) show a completely excavated skeleton, sometimes filled 

 up here and there, or with the inner walls of tlie skeleton 

 lined with a little calc-spar or brown spar ; nevertheless the 

 originally siliceous nature of the skeleton must be maintained, 

 because silicification and excavation of the trabecule seem to 

 stand in definite relation and reciprocal action to one another. 



As regards the Hindioi especially, it appears very impro- 

 bable that groups so widel}^ separated genetically as the Calci- 

 spongige and Bilicispongiee should so perfectly imitate one 

 another in their skeletal parts as would be the case with the 

 Hindice as supposed Calcisponges and the true siliceous 

 Tetracladina3. However the secretion of the spicules from 

 the syncytium may take place, the skeleton-secreting structures 

 are comparable to organs ; and it does not appear admissible 

 without further evidence that these quasi -organs, which have 

 to perform such a different work in siliceous and calcareous 

 sponges, should bring forth products so surprisingly similar, 

 not only individually but also in their mode of union, as 

 would be the case here. In the Calcisponges there never 

 occurs such an intimate contact and such a mode of union of 

 the spicules as is produced in Ilindia and the Tetracladina by 

 the dilated extremities of the arms, furnished as they are with 

 granules, nodules, teeth, and pads. 



The supposition of the originally siliceous nature of all 

 these skeletons also finds important support in the circum- 

 stance that in quite indubitable siliceous sponges, namely the 

 Hexactinellida3 from the Pliocene of Bologna, the skeleton, 

 as Manzoni has proved *, is excavated in a precisely similar 



* Manzoni, ' La struttura microscopica delle spugue silicee del mioceue 

 medio della provincia di Bulogna e di Modeua :' Bologiia, 1882. 



