_ 38 - 



arms, which are extended nearly at right angles to the shaft. 

 Length 2,07 mm. ; thickness of shaft 0,2 mm. and extension 

 of the head rays 1,08 mm. Very rare. 



Tethya? sp. 



(Plate III, fig. 8). 



Simple robust trifid spicule, with a very stout, short, ra- 

 pidly tapering shaft, on the summit of which are three short 

 cylindrical rays, directed forwards at an acute angle with the 

 shaft. Length of shaft 1,95 mm.; thickness 0,27 mm.; ex- 

 tension of head rays 0,517 mm. This spicule is distinguished 

 from the preceding by the arms being longer, not so stout 

 and also directed forwards. Rather rare. 



These three last described spicules (Plate III, figs. 4, 5, 8) re- 

 semble each other in having comparatively short and robust shafts 

 and cylindrical, stout, simple, head rays, at the same time each 

 form is distinctly marked off from the others by differences 

 in the shaft and disposition of the head rays, so that they 

 may have belonged to as many species of sponge. This form 

 of spicule differs considerably from the «zone» spicules of 

 existing sponges of the genera Geodia, Stelletta and Tethya, 

 nevertheless their affinities appear to approach nearer to the 

 spicules of this, than of any other group of sponges. 



Siliceous Globules and Globo-stellates of Geodia 

 and allied Genera. 



(Plate I, figs. 25, 26. 27). 



There are present in the contents , of this Horstead flint, 

 three different forms of the small siliceous bodies which are 

 so abundant in the dermal crust of the recent sponges belonging 

 to Geodia and its allied genera. The first of these (fig. 25) 

 are small bodies, either nearly spherical or ellipsoidal in form, 

 and showing, in many examples, an irregularly shaped cavity 



