78 BRITISH FOSSIL SPONGES. 



Spicules op Fossil 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 " Kalksckwamme," 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 1 has described 

 fusiform spicules in Elasmo stoma 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. In the 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, 2 Corn- 

 wall, and at Goes in Holland, and forming the dermal layer of Sestrostomella 

 clavata, 3 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, i). In spicules of this type, 

 which are abundantly present in the fibres of Tremacystia, Hinde, Corynella, Zitt., 

 and Rhaphidonema, 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, to). 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, n). 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- 



1 ' Palaeontographica,' Bd. xxix, p. 11. 



2 ' Quarterly Journ. Geol. Soc.,' vol. xlii, p. 214. 



3 ' Ann. and Mag. Nat. Hist.,' vol. x, 1882, p. 202, pi. xii, fig. 25. 



