146 SILICEOUS SPONGES. 



The distinctive character of the sponge is the great thickness of the walls, which are 

 composed of a continuous mesh, not arranged in a series of folds as in Pachytei- 

 chisma. Neither can the walls be differentiated into a relatively thin true wall and 

 a posterior supplemental skeleton as in Staur07ieina, for the only difference between 

 the inner and the deeper portions of the wall consists in a gradual transition from a 

 regular to an irregular disposition of the spicular mesh. 



The systematic position of this genus is uncertain. In the characters of the cloacal 

 surface it resembles sponges of the Euretidae family ; but the octahedral nodes and the 

 slight development of the canals exclude it from this group. In some respects the 

 wall-structure resembles that of the genus Biplodictyon ; and it may provisionally 

 be associated in the same family with this genus. 



ScLEROKALiA CuNNiNGTONi, Hinde, n. sp. (Plate XXXI. figs. 3, 3«, 3 6, 3 c.) 



The only example resembles a bird's nest in form. The walls are thickest in the 

 lower and basal portions, the upper margins are rounded. The specimen is 85 mm. 

 in height and 145 mm. in width. The extreme thickness of the wall is 50 mm. The 

 apertures of the inner surface of the wall are about 1"5 mm. in length and about 

 2-5 mm. apart. The mesh-interspaces are circular or oval ; the nodes are from "45 to 

 ■5 mm. apart. The pores between the rays forming the spicular nodes are very 

 minute, so that their octahedral characters are not readily perceptible. The spicular 

 arms are about T2 mm. in thickness. 



The spicular mesh is only partially preserved ; in some places the rays or arms 

 have been enveloped in a thin peUicle of silica, which, by the subsequent dissolution 

 of the inclosed spicules, forms hollow casts of the mesh. 



With this species I have associated the name of Mr. William Cunnington, F.G.S., 

 from whom the Museum has obtained its most valuable examples of sponges from 

 the Chalk and Green Sand of Wiltshire. 



Distribution. Upper Green Sand : near Devizes, Wiltshire. The type specimen 

 belongs to the Jermyn-Street Museum. 



Family C(ELOPTYCHID^, Zittel. 



Genus CCELOPTYCHIUM, Goldfuss, 1826-33, 



CffiLOPTTCHiUM AGAKicoiDES, Goldf. (Plate XXXI. figs. 4, 4 a.) 



1826-33. Caloptychmm agaricoides, Goklf. Petrcf. 1 Th. p. 31, t. 9. f. 20. 

 1833. Cceloptychium agaricoides, S. Woodward, Geol. of Norfolk, t. 4. f. 19. 

 1841. Cceloptychium agaricoides, F. A. Roemer, Nordd. Krcide, p. 10, t. 4. f. 5. 

 1876. Cceloptychium agaricoides, Zitt. Ucbcr Cceloptychium, Abhand. k. bayr. Ak. II Ci. 

 Bd. 12, Ab. 3, p. 59, t. 3. f. 1, 4, 5, 6, 10, 11, 13 ; t. 4 A. 



