80 BRITISH FOSSIL SPONGES. 
though their detached spicules are sometimes sufficiently numerous to form whole 
beds of rock, as in the Upper Greensand’ 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 laminz. Ina few instances Sponges of these 
groups have been enclosed in flints, so that the spicules retain their natural 
arrangement. In Pachastrella, O. 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. 
Lithistide.—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 move 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 Remains in the Lower and Upper Greensand of the South of England,” 
‘ Phil. Trans.,’ part ii, 1885, p. 408. 
