7G Harvey B. HoU — On Fo.<4l Sjmiges. 



facility with which the pores open and close, to admit or check 

 the inciirrent 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 focts which have been held 

 to militate against the 'possession of such a structure. That the sup- 

 porting tissue in certain ]-ccent species becomes closer and otherwise 

 modified at the surface is clearly ascertained, and in some species there 

 is a crusticular layer of embedded ovaries, abounding in minute spicula, 

 beneath the surface of thesarcode. There is likewise a sponge common 

 on the coast at Tenby, in which, in some individuals, 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 upward 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 membi'ane as distinct from the 

 sarcode. That this soft structure may, under favorable 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 Zoantha- 

 rla.] It is more than probable that this structure is nothing more 

 than the cast of the impi'ession or mould of the outer surface of the 

 sarcode of the sponge, perhaps slightly thickened, but it is not con- 

 stantly present even in the same species. For example, there is a 

 small cylindrical sponge, not infrequent in the Coral Rag 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 fossilization, and is sometimes probably the cast of the 

 outer surface of the sarcode which has left its impress on the mould ; 

 that it is absent in the earlier and gi'owing stages of the sponge, and is 

 not constantly present in the matured individuals of those species in 

 which it occurs ; and, moreover, that it sometimes results from the 

 contact of foreign bodies, in consequence of the increased density of 

 tissue which such bodies are apt to produce. Its value, therefore, even 

 as a specific character, is not great. 



Simple as DeFromentelle's arrangement may at first sight appear, it 

 is open to the objection that it is based upon characters that are not 

 always very constant, or very well defined, and are liable to graduate 

 from one into another. Moreover, it unites in one genus or species 

 individuals which, having a very close similarity in external appear- 

 ance, are totally different in the organization of the skeleton, and, on 



