Reviews — Br. G. J. Hinde's Fossil Sponges. 135 



ticularly the deeper-formed deposits of limestone and chalk, they 

 are more rare in the Upper Greensand. (But Manzoni records 

 them as comparatively numerous in the Miocene of Italy associated 

 with shallow-water organisms.) 



The Calcispongice are most abundant in arenaceous or shallow- 

 water deposits and thus resemble in habitat the living members of 

 this order. 



Dr. Hinde furnishes us with much interesting and valuable infor- 

 mation as to the alterations produced by fossilization in the structure 

 of sponges, a subject of the utmost importance in dealing with these 

 organisms, when their classification (as already stated) depends 

 entirely on their skeletal structures, not upon the modified forms of 

 the sponge-mass. He points out that " not only does all the soft part 

 of the sponge disappear, but that even the mineral portion is seldom, 

 if ever, in the same condition as in recent spicules ; the amorphous 

 silica and calcite have been replaced by crystalline silica and crys- 

 talline calcite, as well as by peroxide of iron and iron pyrites ; whilst 

 not infrequently the entire mineral structure has been dissolved and 

 I'emoved, leaving the empty moulds of the spicular skeleton in the 

 matrix. As a result of these changes, siliceous sponges now occur 

 with skeletons of calcite, and calcareous sponges with fibres com- 

 posed of silica. These changes are intimately connected with the 

 character of the strata in which the sponges are imbedded, but the 

 causes producing them have not up to the present been satisfactorily 

 determined. In general the sponges in calcareous strata have under- 

 gone the greatest alteration, the siliceous structures being replaced 

 either by calcite or iron peroxide, or dissolved away altogether, 

 whilst the structures of calcareous sponges, in common with the 

 shells of molluscs in the same strata, are oftentimes repla(;ed by silica. 

 In arenaceous or glauconitic strata, on the other hand, the changes, 

 whether of siliceous or calcareous sponges, have been much less 

 extensive than in strata of a calcareous character" (p. 4). 



"The first step," says Dr. Hinde, "in arranging a series of fossil 

 sponges in natural order is to ascertain the characters of the spicular 

 skeleton ; and as in the majority of examples no spicular structure 

 is preserved on the outer surface, it is necessary to make a section 

 through the sponge in order to discover, if possible, any indications 

 of structure in the interior. It sometimes happens that all traces 

 of the spicular skeleton have disappeared throughout the central 

 portions of the sponge, as well as on the outer surface ; and in this 

 case the systematic position of the sponge remains somewhat con- 

 jectural. But even when all structure has disappeared in the sponges 

 of certain horizons and localities, we oftentimes find the same 

 sponges from the cori'esponding strata in other places with their 

 skeletal structures in good preservation." Thus the sponges of the 

 Upper Chalk of Flamboro' and the southern counties of England, in 

 which merely the outer form and canal structure is retained, can 

 be determined by comparison with those from the same geological 

 horizon in North Germany, in which the spicular skeleton remains 

 intact (p. 14). 



