14 Transactions of the Society. 



wrinkled (pi. I., fig. 10.). The specimens are now free, but the 

 wrinkled character of their bases seems to indicate that they may- 

 have been attached to some foreign body during life. From the 

 surface elevations deeply impressed branching canals radiate down 

 their sides. 



Typical examples of Porosphwra pilcolus are thimble- or in- 

 verted cup-shaped, with thick walls and deeply concave bases ; 

 the basal hollow is lined with a spicular dermal layer and shows 

 successive growth-rings or bands. The upper surface of this 

 species resembles that of P. globularis (pi. I., figs. 20, 21). In 

 P. patelliformis, the outer form resembles that of a limpet, the 

 basal cavity is wide, the walls are comparatively thin, and it is 

 furnished with a distinct basal dermal layer (pi. I., figs. 22-26«) 

 In yet another form which I have named arrccta, the sponge is 

 like a small, upright tapering pillar with a concave base (pi. I., 

 figs. 27-28«). In some specimens the base may probably have 

 been attached originally to some other body. 



VI. Skeletal Mesh. 



The skeleton of Porosphara is of a stony character ; in thin 

 sections under the Microscope it appears to be made up of a finely 

 porous mesh-work of continuous anastomosing fibres, in which, 

 however, little structure can now be distinguished. The nature 

 of the mesh-fibres is better shown on the surface of specimens 

 obtained directly from the Chalk. These, when carefully cleaned 

 from the matrix, exhibit under a strong lens or a Microscope a 

 multitude of minute, projecting spines or rays, which also can be 

 recognised by a rasping sensation when the finger is rubbed over 

 the surface. On close examination, each of these projecting rays 

 can be seen to spring from the central junction of three other short, 

 generally recurved rays, and they are, in fact, the apical rays of 

 four-rayed spicules similar to those in Plectroninia and Petro- 

 stroma. The three sub-equal, short facial rays of these spicules have 

 truncate ends, which are fused or welded to the surfaces of the 

 adjoining spicules, in such a manner as to form the mesh-fibres 

 which delimit the radial canals (pi. II., figs. 6, 0). 



The nature of the skeleton of Porosphccra in the early stages of 

 growth is admirably shown in a small specimen of P. glolularis, 

 about 1*25 mm. in diameter, which has been preserved in a flint from 

 the Upper Chalk of Kent (pi. II., fig. 1). The specimen was dis- 

 covered by Mr. H. Muller, of Eltham, to whom I am indebted for 

 the opportunity of studying it. The apical rays are very prominent, 

 and the facial rays are already firmly fused to those of adjacent 

 spicules. By further surface growth these prominent apical rays 

 would be partially surrounded by and welded to the facial rays of 

 the succeeding layer of spicules above them, and would thus be 



