SO BRITISH FOSSIL SPONGES. 



though their detached spicules are sometimes sufficiently numerous to form whole 

 beds of rock, as in the Upper Greensand 1 of the Isle of Wight and elsewhere. 



Of the few fossil forms which have been discovered may be mentioned the 

 genus Haplistion, Young and Young, from the Carboniferous strata of Scotland, in 

 which the acerate spicules are closely arranged into anastomosing fibres (Plate V, 

 figs. 1, 2) ; Opetionella, Zitt., from Cretaceous strata, in which the large acerate 

 spicules are disposed side by side to form a thick lamina, and Scoliorhaphis, in 

 which the spicules form meandriform laminse. In a few instances Sponges of these 

 groups have been enclosed in flints, so that the spicules retain their natural 

 arrangement. In Pachastrella, 0. Schmidt, the spicules are loosely aggregated 

 together, without any apparent order, so as to form a thick wall. Entire examples 

 have been preserved in the Upper Chalk of Flamborough, Yorkshire. In these 

 and the other Sponges mentioned, the spicules, originally only held together by 

 the spongin, are now for the most part lightly cemented and fused together by a 

 secondary deposition of silica, produced during fossilization from a partial solution 

 of the spicules themselves. 



The polyaxile stellate, globostellate, and discoid spicules, which form the 

 dermal layer in some tetractinellid Sponges, are, like the skeletal-spicules in these 

 Sponges, merely held together in their natural position by the soft structures of 

 the Sponge, and they have hitherto only been met with detached in the fossil 

 condition. 



Lithistidce. — In this sub-order also the skeletal-spicules in their natural condition 

 are not organically united, but the terminal extremities of the spicular rays are 

 firmly linked together in various ways, so as to produce a resistant structure ; 

 owing to which, and to the further fact that the walls of these Sponges are often 

 of considerable thickness, entire examples are of frequent occurrence in the fossil 

 state. Their detached spicules are, however, extremely abundant in certain strata, 

 thus showing that in many instances the union of the spicules has not been suffi- 

 ciently strong to resist disintegration of the skeleton. 



In the Tetracladina family the spicules are united together by the interlocking 

 of the tubercular extremities of the twig-like subdivisions of their rays with those 

 of proximately adjoining rays, so that a prominent rounded or oval node is pro- 

 duced by their combination, as in Callopegma (Fig. 5, d). These twig-like 

 extensions of the rays are so intricately intertwined together that it is almost 

 impossible to separate them without fracture of the more delicate portions. The 

 skeletal mesh thus formed has irregular oval or polygonal interspaces. The spicular 

 rays bounding the canals are deflected so as not to protrude into their channels, 

 but there does not appear to be a specially modified membrane lining their walls. 



1 " Beds of Sponge Eemains iu the Lower and Upper Greensand of the South of England," 

 ' Phil. Trans.,' part ii, 1885, p. 403. 



