SKELETAL STRUCTURES. 



77 



ceding types, and as there are no traces of their derivation from any of them it 

 is necessary to place them in a distinct group. 



There are also some other forms of fossil multiaxial siliceous spicules, most of 

 which, if not all, belong to the dermal layer of various Sponges, and differ very widely 

 from the skeletal-spicules of the body of the Sponges to which they belong. Of 

 these the most common are the kidney-shaped or sub-spherical forms which build 

 the outer crust of Geodia. These have been supposed by Oscar Schmidt to consist 

 of a radial agglomeration of minute uniaxial spicules (Fig. 7, b, c). Others are the 

 so-called stellate (Fig. 7,/,) and globo-stellate spicules (Fig. 7, d, e), in which a 

 variable number of conical rays project either from a central point or from a 

 rounded nucleus. Another widely-distributed spicule has the form of a delicate 



J\fX200 



Fig. 7. — Different forms of fossil siliceous heteractinellid and multiaxial spicules and also spicules of 

 fossil calcisponges. (a) Spicule of Asteractinella expansa, with twelve unequal rays proceeding from 

 a common centre, (b) Kidney- shaped spicule of the dermal layer of Geodia. (c) Another dermal 

 spicule of Oeodia, showing its composition of apparent spicular rods. (d) Globostellate spicule in 

 which there are traces of canals in each of the rays, (e) Globate spicule with a surface covered by 

 minute spines. (/) Stellate spicule, probably of the dermal layer of a tetractinellid Sponge. 

 (g) Thin elliptical disc with canals radiating from the centre ; probably a dermal spicule, named 

 Stellettites callodiscus, Carter. (h) A three-rayed equiangular and equiradiate spicule of fossil 

 calcisponge. (»') A three-rayed sagittate spicule, with two equal, paired rays, and one ray, the basal, 

 shorter than the other two. (J, m) Three-rayed spicules, with the basal ray only slightly developed ; 

 from the fibre of Tremacystia D'Orbignyi, Hinde. (Jc) Four-rayed sagittate spicule with blunted 

 apical ray, from the same Sponge. (I) A four-rayed spicule, showing the apical ray. (») Three 

 rayed (tuning-fork) spicule, in which the paired rays are nearly parallel with each other. 



elliptical disc, bordered by minute flask-shaped cavities, and a series of canals 

 radiating from the centre to near the margin. This has been termed Stellettites 

 callodiscus, Carter, but it is not yet definitely known whether it really belongs to a 

 tetractinellid Sponge. 



