78 BRITISH FOSSIL SPONGES. 
Spicunus or Fosst CALCISPONGES. 
The spicules of calcisponges are much less varied in form, and likewise of much 
smaller proportions generally than the skeletal-spicules of fossil siliceous Sponges, 
The fossil examples, so far as at present ascertained, differ but slightly in form 
and size from those of recent Sponges of this group, but there is a special difficulty 
in studying them, since it is an extremely rare circumstance to obtain them detached, 
and their individual outlines can seldom be seen complete in microscopic sections 
of the fibres in which they are interlaced. Notwithstanding this drawback many 
of the modifications of the chief types of these spicules, which have been so 
exhaustively described by Haeckel in his “ Kalkschwamme,” can be recognised in 
fossil calcisponges. 
1. Simple uniaxial spicules. These in microscopic sections can scarcely be 
distinguished from portions of three-rayed spicules, more particularly when the 
basal ray of these latter is only slightly developed. Dunikowski! has described 
fusiform spicules in Elasmostoma and Pachytilodia with pointed and rounded 
extremities, and Pharetrospongia Strahani, Sollas, seems to be entirely composed 
of straight or slightly curved uniaxial spicules. 
2. Three-rayed spicules. Inthe simplest form or “ regular” spicules, the rays 
are in the same plane and the rays and angles are equal (Fig. 7, h). These have 
been met with in a detached condition in tertiary deposits at St. Erth,’ Corn- 
wall, and at Goes in Holland, and forming the dermal layer of Sestrostomella 
clavata,* Hinde, from the Upper Greensand of Warminster. In the “ sagittate” 
spicules two of the rays are paired and equal, and the third or basal ray may be 
either longer or shorter than the other two (Fig. 7,7). In spicules of this type, 
which are abundantly present in the fibres of Tremacystia, Hinde, Corynella, Zitt., 
and lthaphidonema, Hinde, the paired rays form a very open angle or a regular 
curve, in the centre of which on the convex side is a small blunt projection repre- 
senting the basal ray (Figs. 7, j, ). In another abnormal three-rayed sagittate 
spicule, occurring in Sestrostomella, Zitt., and also in the recent Leucetta pandora, 
Haeck., the paired rays are nearly parallel with each other and the basal ray extends 
backwards, so that the spicule is similar in form to a tuning-fork (Fig. 7,2). In 
the “irregular” three-rayed spicules all the rays and the angles are unequal. 
The axial canal in the spicules of fossil calcisponges can very rarely be detected, 
but traces of its presence have been noted by Dunikowski in spicules of Elasmo- 
‘ Paleontographica,’ Bd. xxix, p. 11. 
1 
* *Quarterly Journ. Geol. Soc.,’ vol. xlii, p. 214. 
3 «Ann, and Mag. Nat. Hist.,’ vol. x, 1882, p. 202, pl. xii, fig. 25. 
