22 



a cone and a cylinder. Outer surface originally smooth. The 

 interior canal is frequently preserved, it is of a conical form 

 corresponding to the shape of the spicule ; in some instances 

 it extends but a short distance from the head, in others it is 

 prolonged nearly to the point of the spicule. These conical 

 spicules are very abundant and very regular in form. They 

 vary in length between 0,495 mm. and 0,832 mm. and in 

 width of the head or thickest portion between 0,135 mm. 

 and 0,225 mm. 



I have also found a single specimen of a spicule nearly 

 alike in form to these small cones but characterized by the 

 surface being completely covered with small blunt spines 

 (fig. 22). No canal can be distinguished. The specimen is 

 0,45 mm. long and 0,157 mm. wide. 



The only instance of spicules of this conical form, which 

 I have met with, is given by Mr. Carter as a transitory stage 

 of the dermal globular crystalloids of Pachymatisma JoJm- 

 stonia, Bowerbank (Ann. Mag. Nat. Hist. S. 4, Vol. 4, p. 9, 

 PL II, fig. 1 8 c). The resemblance is however merely in the 

 outer form, for Mr. Carter states as regards these bodies, that 

 on no occasion has he been able to detect a central cavity in 

 any stage of their development, whereas the interior canal is 

 clearly exhibited in the Horstead spicules. As regards their 

 respective dimensions I am unable to make a comparison, for 

 the scale to which Mr. Carter's figure has been drawn, is not 

 stated. These conical spicules appear to be related to the 

 cylindrical spicules present in some species of Reniera and I 

 have therefore placed them provisionally under this genus. 



These conical spicules have been found, though rarely, 

 in the North of Ireland (Wright; op. cit. p. 88, PL II, fig. 3) 

 and I have also discovered them in material from Coesfeld, 

 given me by Prof. Zittel. The spinous form also is pre- 

 sent in the Upper Chalk Strata of Coesfeld. 



