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iS) 
Ww 
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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 Fohn- 
stonta, Bowerbank (Ann. Mag. Nat. Hist. S. 4, Vol. 4, p. 9, 
Pl. I, fig. 18 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. 
