JSarve]! B. Holl—Gn Fossil Sjionges. 75 



or .slightly wrinkled membrane, wliich has been regarded by D'Orbigny, 

 DeFromen telle, F. A. Koemer, and others, as analagous t(^ the epitheca 

 of the Zoantharia ; and the occurrence of this epitheca has been held 

 to be an additional evidence of the stony nature of the sponge skeleton. 

 When examined microscopically, by means of thin sections, howeyer, 

 it appears that this epitheca is due to the filling up of the interstices 

 of the superficial parts of the sponge, Avhich in the situations hi which 

 it exists is finer and more condensed than elsewhere. This greater 

 density of the surface may be seen in many recent sponges, the super- 

 ficial portions of the tissues being closer and finer than that of the 

 interior, which Avas foi'med during an earlier and more active period 

 in the growth of the sponge. But either from having arrived at 

 maturity, or at a period when the growth was temporarily arrested, 

 for in some sponges the growth is intermittent, or, as appears sometimes 

 to be the case, from some local cause, the tissue at the surface assumes 

 a closer arrangement. Thus, in the common Halidiondria jmnicea, the 

 surface over greater or less portions frequently presents a condensed 

 appearance, with scarcely any visible interspaces, the outer superficial 

 portion being made up of a densely matted layer of spicula, placed for 

 the m4)st part parallel to the surface ; and the same is true of many 

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

 sp mges a simdar 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 inter- 

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

 carbonate of lime. Thus, a species of Cupuh^ponrjia, 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 only occurs 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 fri'^tion 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, weie it 

 really of the nature of a true epitheca, as the Zoantharia, it would be 

 difficult to comprehend how the functions of the animal were carried 

 on. [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 opinion appears to exist, the 



