FROM THE SILURIAN FORMATION OF CANADA. 



257 



exactly similar spicule over on its head, so that its shaft projected 

 vertically ; the four capitular rays could then all be seen in one view. 

 The axial canal of the shaft is prolonged upwards past the common 

 origin in the head, and, no doubt, once extended into a short vertical 

 spicule ; indeed the companion spicule shows as much. In addition 

 to this axial spine, containing a continuation of the canal, the head 

 is ornamented by conical tubercles given off in the same direction 

 from the upper surface of the capitular rays (as shown in the figure). 



I take this spicule to have projected radially from the surface of 

 the sponge-body, the shaft being directed inwards and the capitular 

 rays expanding parallel to the surface of the body, as in the similar 

 spicules of Holtenia and Mossella ; it has not the appearance of an 

 anchoring spicule. 



Figs. 3 and 3a. These two drawings represent the same specimen, 

 the upper one taken before immersion in a liquid medium and by re- 

 flected light, the lower one by transmitted light when mounted in 

 Canada balsam. They are magnified 104 diameters : so that, if 

 represented on the same scale as the other figures, they would not be 

 quite so large as the spicule represented in fig. 6. The noticeable 

 feature is the presence of spines on the rays, directed in all cases 

 towards the ends. The rays were evidently at least five in number ; 

 probably there were six. 



Fig. 4. The four rays of this spicule are directed downwards 

 below the plane of the paper ; near their origin they each give rise 

 to a large bifid spine inclined upwards away from the plane of the 

 paper. It is probable that this specimen once formed the head of a 

 spicule like fig. 2. 



' Fig. 5. This spicule (X 104) is chiefly remarkable for its spinose 

 rays : on the longest but still broken ray three spines are given off 

 from one side, and a fourth, represented in plan by a circle, from 

 the adjacent side at right angles. 



Fig. 6. The proximal half of the rays is thicker than the distal 

 end, which is long, slender, and directed downwards, the proximal 

 half lying in the plane of the paper. The thicker part of the rays is 

 spined — the spines (so far as one can judge from the three which 

 remain on one of the rays) being arranged spirally, and two of 

 them arising near the origin of the slender extremity. 



Fig. 7. This represents a fragment of a cylindrical spicule magni- 

 fied 104 diameters, with the axial canal filled up with silica, which 

 now projects at both ends like an acerate spicule run through the 

 middle. 



Fig. 8 is also a fragment of a cylindrical spicule ; it shows the 

 hemispherical pits which have been eaten out of its surface. 



Fig. 9. This is one of the most frequently occurring forms ; it is a 

 large simple sexradiate — the rays having a cylindrical form, only 

 slightly tapering towards their broken extremities. The axial 

 canals are regular and clearly defined. This is probably one of the 

 staple, body-spicules of the sponge. 



Fig. 10 is a smaller spicule of the same kind. 



Fig. 11. This is less like a Hexactinellid spicule than the others, 



