140 SILICEOUS SPONGES. 



Genus CAMEROSPONGIA, BOrhigny, 1847. 



Camerospongia subrotunda, Mant. sp. (Plate XXX. fig. 3.) 



1822. Choanites subrotundus, Mant. Foss. of the South Downs, p. 179, t. 15. f. 2. 



1831. Choanites subrotundus, Benett, Cat. Org. Rem. Wilts, t. 16. f. 1. 



1848. Cephalites constrictus, Toulm. Smith, Ann. & Mag. Nat. Hist. vol. i. p. 292, t. 15. f. 1. 



Sponges depressed globate in form, growing either singly or in small groups, and 

 not infrequently in a single series of two or three individuals closely attached 

 together. The sponges appear to have been attached by a few divergent rootlets 

 given off laterally ; in the compound examples only the first sponge in the series 

 appears to possess the rootlets. Separate individuals vary between 15 and 35 mm. 

 in breadth, and 6 and 13 mm. in height. The upper surface is slightly rounded, 

 oftentimes with a gentle depression towards the centre. The aperture is circular or 

 oval, from 3'5 to 9 mm. in width, with an elevated margin, encircled by a shallow 

 furrow. 



Nearly the entire upper surface and sides are covered by the smooth siliceous 

 membrane. The under surface is uneven, with a few circular apertures between 

 the ridges formed by the folded walls. The convoluted wall-plaits of the interior 

 are slender, and composed of a spicular mesh with lantern nodes ; the nodes are 

 •375 mm. apart. 



The subglobate form and the social mode of growth distinguish this species from 

 others of the genus. 



Distribution. Upper Chalk : Lewes, Sussex ; Charing, Kent {coll. Mantell, T. Smith). 

 Also from the Spongarian zone of the Chloritic Chalk (T«^e) = Upper Chalk : White- 

 head, near Belfast [Jermyn-Street Museum). 



Camerospongia capitata, Toulm. Smith, sp. 

 184.8. Cephalites capitatus, T. Smith, Ann. & Mag. Nat. Hist. vol. i. p. 288, t. 14. f. 11. 

 1878. Cephalites capitatus, Quenst. Petref. Bd. 5, p. 498. 



Sponges inverted conical in form, supported on a slender cylindrical stem, with 

 divergent rootlets. The summit is gently convex, the margin is subacute. From 

 the summit the body rapidly tapers to the base. The typical specimen is 21 mm. in 

 height by 31 mm. in width; the stem is 11 mm. in length. The summit-aperture is 

 circular, 11 '5 mm. in width, its margin is thickened, and slightly elevated. 



The smooth enveloping membrane only covers the summit ; the sides exhibit a 

 reticulate aspect, formed by wrinkled ridges with circular or irregular interspaces. 

 A sub cylindrical or funnel-shaped cloacal cavity extends from the summit to the 

 base, though it does not appear to have possessed a definite boundary wall. The 

 interior wall-plaits are loosely convolute; they are about -1 mm. thick. The nodes 

 of the spicular mesh are '45 mm. apart. 



