Dr. G. J. Hinde on the Genus Illndia. 75 



This resemblance is so palpable that even Prof. Duncan 

 originally described Hindia as a lithistid sponge. But as the 

 name lithistid was applied bj Oscar Schmidt only to sponges 

 with siliceous skeletons, it is therefore a decided misnomer 

 thus to terra Hindia, when it is regarded by Duncan as a 

 calcareous sponge. If it is really a calcisponge it should 

 stand alone as the only extinct representative of a distinct 

 order in that group, since there is no other known calcisponge 

 with spicules or a spicular structure at all resembling those of 

 Hmdia, 



Prof. Duncan finally pleads, in the September number, 

 that the former existence of a mimetic series of calcareous 

 sponges is within reasonable distance of the truth, for who 

 amongst us is to limit Nature as regards possibilities? (p. 228). 

 But in determining the character of this fossil sponge, regard 

 should first be taken for i\\t facts of Nature, and if, according 

 to all analogies, these point to the siliceous origin of Hindia, 

 it is altogether beside the point to suggest the possibilities of 

 Nature to produce a mimetic series of calcareous sponges, or 

 to surmise that the group may have become extinct or merged 

 into a higher form, as the parent of Zoantharia perforata. 

 When such rash speculations depend mainly on the supposed 

 fossilized filaments of an alga *, it is not surprising if they 

 prove to be far from within reasonable distance of the truth. 



I am able to confirm the careful descriptions of Hindia 

 given in Dr. RaufF's paper in nearly every respect. The 

 microscopic sections studied by this author showed more 

 clearly the junction of the spicules than those at my dis- 

 posal, and he has established the observation of Duncan that 

 there are not more than four rays in the elementary spicule, 

 whereas I thought it probable that the number might have 

 varied from four to six f. He has also shown that the union 

 of the spicules does not take place by the junction of the 

 frilled ends of their rays with each other, as stated by Duncan 

 and accepted by myself, and he explains Duncan's figures 

 (' Annals,' 1879, vol. iv. pi. ix. figs. 1 a, 2, 2) by supposing 

 that they have been drawn from a transverse section of the 



* Prof. Duncan's statements respecting this fossil alga, Pakeachlya 

 perforans, require for their acceptance an unlimited faith in the possi- 

 bilities of Nature. Not only does it exist in these Silurian sponges, but 

 it has bored cavities in the scales of Cretaceous fishes, in the hard parts 

 of both fossil and recent corals and shells, and, mirabile dictu, the same 

 species still exists, and works its ravages on the bodies of our common 

 house-flies — this is the aerial form of the Achhja ! Who would have 

 imagined a direct genetic connexion between the parasite of a Silurian 

 marine sponge and that of a house-fly, dead on the wall ? 



t Cat. Fdss. Sponges Brit. Mus. p. o7. 



