70 Harve-ij B. Holl— On FusmI Sponges. 



manner the mould may (36 refilled 'with carbonate of lime. On the 

 other hand, an originally calcareous sponge may become converted into 

 a siliceous fossil, as we see in the case of the mollusca of the Great 

 Orme's Head and elsewhere, and that an interchange of this kind has 

 actually taken place seems necessary to explain the fact that siliceous 

 and calcareous s])onges are not usually found associated in the same 

 spot. 



That the calcareous sponges, those of the English Oolites for instance, 

 are merely casts of the original structure may be shown in another 

 way. If, then, sections of the fibers may be made sufficiently translu- 

 cent for the employment of a quarter-inch power of the microscope, it 

 will be seen that the fiber has often the asbestiform structure radiating 

 at right angles from a central axis, i^eculiai'ly a mineral arrangement, 

 and especially of carbonate of lime. Tiie structure of true sponge 

 fiber on the contrary is concentric. The preservation of the sponge, 

 therefore, in its fossil state, depends very much on the nature of the 

 sediment in which it is embedded, and in the mode of its entombment. 

 Hence they are met with but rarely in stiff' ai-gillaceous deposits ; and 

 although abundant in Mesozoic times, they are absent from all the 

 clayey members of the series, such as the clays of the Lias, and the 

 Oxford and Kimmeridge clays ; yet they occur in continental beds of 

 corresponding ages, but differing in litliological character. At the 

 same time it is possible that the muddy wMtors of the seas in v.'hich 

 these deposits were thrown down, may have been ill adapted to the well- 

 being of the sponge, nevertlieless they are met with in the Linguia 

 Flags and in the Upper Silurian Shales. 



The condition of the sponges of the gravel pits of Farringdon is re- 

 markable, and may, perhaps, tend to throw some light on the mode in 

 which fossilization of the sponge takes place in calcareous and such 

 sandy deposits as contain lime, and help to explain, in some manner, 

 the absence of compression in the fossil. Every tw'ig of the sponge 

 presenting a free surface throughout its entire thickness, is invested by 

 a thin coating of minute dog-tooth crystals of carbonate of lime, form- 

 ing a complete crust over the fiber, much in the same manner as moss,- 

 etc, is encrusted by a calcareous spring, These crystals seldom ex- 

 ceed yiy of an inch in height, the average being about y-Jy-g- of an 

 inch, or e^-en less ; and on their sui'face they are slightly tinged by 

 peroxide of iron. All the other fossils of the same locality are simi- 

 larly coated with these minute crystals, even to the interior of the cells 

 of the Polyzoa. When slightly acted upon by dilute acids, the crys- 

 talline layer is removed, and the cast of tne sponge fiber exposed ; 

 but if this action of the acid be contamed, the whole is dissolved 

 usually without leaving any trace of siliceous sjjicula. 



