38 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery X. are closely apposed to the axis and branches of adjoining 

 spicules so as to form loosely arranged fibres or an irregular 

 meshwork (Fig. 12 a, h) ; the Cretaceous Verruculina 

 (Fig. 17) is characteristic. 



Sub-Class II. — monactinellida. Demospongiae 

 with monaxon spicules. 



The chief modifications of this simple type of spicule 

 are shown in the accompanying figure (Fig. 14 a-q). With 



Fig. 14. — Fossil sponge spicules : Silicispongiae, Monactinellida. a-q are 

 skeletal, r, s, t are flesh-spicules. An open axial canal is shown in /; 

 a closed canal in g. a, e, enlarged 26 diam. ; 6, 100 diam. ; c, f, g, i, j, 

 k, q, 40 diam. ; d, 66 diam. ; li, m, n, o, r, s, t, li4 diam. , I, p, 13 diam. 

 (After Hinde.) 



these are associated smaller flesh-spicules, serving as grapnels 

 in those forms that have a harder outer skin or cortex 

 (Fig. 14 r, s, t). In these latter, constituting the Order 

 Hadromerina or Spintharophora, the spicules are loosely 

 and irregularly arranged. In the other Order, Halichon- 

 drina^ the siliceous spicules are bound into a skeleton by 

 the horny substance spongin. Consequently, though the 

 spicules abound in siliceous deposits, chiefly of Tertiary age. 



Table-cases complete sponges are rarely preserved. Clmlina, Beniera, 

 15, 14. and the freshwater Spongilla are among fossil representatives 



Table-ease of the Halichondrina. The Hadromerina are important to 

 11- geologists from including the boring Clionidae, whose tubes 

 are often seen traversing fossil shells ; indeed they are also 

 active in the disintegration of rocks. 



Class IY. (?) OCTACTINELLIDA. 



Silicispongiae, in which the normal spicule has eight 

 rays, of which six radiate in one plane from a common 

 centre, while the other two radiate from the same centre at 



