32 SILICEOUS SPONGES. 



The upper surface of the body is thickly covered as far as the marginal edge with 

 circular apertures '75 to 1 mm. in width ; in specimens treated with acid these 

 openings appear to be oblique to the plane of the surface. The under surface of the 

 body in the best-preserved specimens seems to be composed of a close dermal layer, 

 which, however, is usually absent, and then the vertical lamellae are exposed. 



The sponge itself is composed of a series of delicate fibrous lamellae extending 

 from the centre to the circumference, having a general resemblance to the septa of 

 corals. These lamellae are about 0-4 mm. in thickness and about 1 mm. apart. 

 Numerous interstitial fibres connect the radial lamellae, so that the aspect of a 

 vertical section of the sponge-wall is that of a labyrinthine fibrous web. In all the 

 specimens the spicules forming the fibres have been so fused together that the 

 individual forms cannot be detected. 



As no description whatever accompanies the figures of the sponges from the 

 Flamborough Chalk given by Phillips in the ' Geology of Yorkshire,' and as the 

 figures are very imperfectly drawn, it is often a matter of great difficulty to deter- 

 mine the forms which they are supposed to represent. Judging by the represen- 

 tation of Spongia capitata, plate 1. fig. 2, it would appear to possess but slight 

 relationship with the Sponcjia plana (fig. 1). In reality, however, the only difi'erence 

 between the sponges thus designated consists in the outer form; the summit of 

 iS'. capitata has not been developed to the same extent as that of »S'. plana. The 

 examples in the Museum collection show a series of gradational forms between those 

 in which the summit of the sponge is scarcely more expanded than the top of the 

 stem, and those with very widely expanded bodies ; and there can be little doubt that 

 Phillips's <S'. capitata should be included in the same species as the S. plana. 



This species seems to be the most common of any of the Flamborough sponges, 

 but it does not appear to have been recognized from any other locality in the 

 English Chalk. 



Distribution. Upper Chalk : Flamborough, Yorkshire. 



Seliscothon explanatds, Rcemer, sp. (Plate II. fig. 5.) 



1864. Chenendopora eaplanata, Rcemer, Palseont. Bd. 13, p. 44, t. 16. f. 3. 

 1878. Seliscothon explanatus, Zitt. Stud. II Ab. p. 54, t. 4. f. 2. 



Body of sponge forming a flat plate-like expansion, apparently circular in outline. 

 No stem has been preserved. The upper surface has a few concentric, slightly 

 marked, rounded ridges. The margins rounded. The only specimen is a portion of 

 a sponge, about 180 mm. in width across the summit and 5 mm. in thickness. 



The upper surface is furnished with a dermal membrane in which are numerous 

 irregularly disposed circular apertures 0"75 mm. in width, and from two to three 

 diameters apart. The under surface appears to be entirely covered with the dermal 

 membrane. 



