Dr. S. B. mil— On Fossil Sponges. 347 



made up of a densely matted layer of spicula placed for the most 

 part parallel to the surface ; and the same is true of many fibrous 

 sponges, as shown by Dr. Bowerbank. In some of the fossil sponges 

 a similar modification of the tissue at the surface appears to have 

 obtained, especially in certain cup-shaped and cylindrical sponges, 

 and in the calcareous fossils in. which this has been the case, the in- 

 terstices, from their extreme minuteness, are more or less filled with 

 carbonate of lime. Thus a species of Cupiilosjjongia, common in 

 the gravel-pits of Farringdon, frequently presents on its interior a 

 smooth surface, described by the late Daniel Sharp as a membrane.^ 

 But if a number of individuals of this species be examined, it will 

 be observed that although it sometimes completely lines the interior 

 of the cup, it more often occurs only in patches ; and that, while 

 some of the interstices are blocked up, there are others that remain 

 open, and this not as the consequence of friction or weathering, but 

 as the result of fossilization. It is, however, in some of the 

 sponges of the Oolite that we see this infilling of the interstices most 

 distinctly, in some of the cylindrical forms especially, the whole of 

 the sponge, except its summit, appearing as though invested by a 

 sheath, but which, were it really of the nature of a true epitheca, as 

 in the Zoaniharia, it would be difficult to comprehend how the 

 functions of the animal were carried on,* It is more than probable 

 that this structure is nothing more than the cast of the impression or 

 mould of the outer surface of the sarcode of the sponge, perhaps 

 slightly thickened, but it is not constantly present even in the same 

 species. For example, there is a small cylindrical sponge, not un- 

 frequent in the Coral Eag, at BuUington Green, near Oxford, in 

 which more or less of this so-called epitheca is met with in some 

 individuals, while the greater number show nothing of the kind. It 

 appears, therefore, that the epitheca is sometimes only a result of 



1 Quart. Joum. Geol. Soc, vol. x., p. 196. 



^ It is not here necessary to discuss the question of a "dermal membrane," as this 

 "would of course perish with the sarcode, and about ■which, moreover, some difference 

 of opinioa appears to exist, the facility with which the pores open and close to admit 

 or check the incurrent streams of water, and the readiness with which the sarcodal 

 mass is repaired after injury, and unites on contact with that of another individual of 

 the same species, are facts which have been held to militate against the possession of 

 such a structure. That the supporting tissue, in certain recent species, becomes closer 

 and is otherwise modified at the surface, is clearly ascertained (see Bowerbank. 

 Ecronemia acervous, Bowerb., MSS,, I.e., p. 173, pi. 28, £ 355 ; also pp. 107, 108, pi. 

 20, f. 309 and 310 ; Halichondria panicea, Johnston, pL 19, f. 303), and in some 

 species there is a crusticular layer of embedded ovaries abounding in minute spicula 

 (Bowerbank, I.e., Pachymatisma Johnstonia, Bowerb,, p. 172, pi. 27, f. 353 ; Geodia 

 Barretti, Bowerb., MS., p. 169, pi, 28, f, 354) beneath the surface of the sarcode. 

 There is likewise a sponge common on the coast at Tenby, in which, in some indi- 

 viduals, the base and for a little distance above it does appear, in the dried condition, 

 to be invested by something like a membrane, which terminates upwards in a well- 

 defined and thickened or slightly wrinkled margin ; the kerato-spicular tissue of the 

 sponge immediately beneath it is more densely reticulated than in other parts of the 

 animal ; but I was unable to satisfy myself that it constituted a true membrane as dis- 

 tinct from the sarcode. That this soft structure may, under favourable circumstances, 

 so impress the mould of the fossil as to produce the appearance described as an 

 epitheca, may be possible ; but this is altogether different from the sclerotic sheath 

 which invests the exterior in the Zoaniharia. 



