W. J. SOLLAS ON PHARETROSPONGIA STRAHANI. 247 



sen ted in PL XI. fig. 6, where a fibre is longitudinally striated 

 throughout except at its rounded termination, where the striae are 

 replaced by circles. 



This is due to the section crossing a fibre which at one part of its 

 course lies in the plane of the transparent slice, but afterwards 

 bends somewhat abruptly at right angles to it, and so presents us 

 with a transverse section of the fibre, in obvious continuity with a 

 longitudinal one (diagram, Fig. 6) ; and as the longitudinal section 



Fig. 6. 



Lateral view of a fibre bending at c, at right angles to the plane of the section, 

 a, b, thus producing the appearance shown in the section Plate XI. fig. 6. 



of the fibre passes into the transverse one, the longitudinal acicular 

 section of the spicules passes correspondingly into the transverse 

 circular one. 



It is noticeable that the spicules are not confined to the axis of the 

 fibre, as in the recent Chalinidse, but are uniformly crowded through- 

 out it ; the spicules, indeed, are the fibre, the granular material which 

 fills the interstices being no greater in amount than that required to 

 fill up the spaces between a number of cylindrical rods closely pressed 

 together. Nor do the spicules differ in size to any important extent ; 

 they are all, so far as one can see, of the same size and shape ; nor 

 do they follow two chief directions, one transverse and the other 

 longitudinal to the fibre, as in the Echinodermata, but are bound 

 together in parallel bundles like the Roman fasces. 



It is difficult to find an example of a complete spicule sufficiently 

 isolated from the rest to determine its dimensions and the nature of 

 both its ends, though there are a few very exceptional cases in which 

 this can be accomplished. At the place, for instance, where two or 

 more fibres anastomose, the contained spicules cross one another at 

 various angles, and so become separately visible to a much greater 

 extent than when they all lie in a sheath side by side (PI. XI. 

 fig. 12, example from a recent sponge) ; occasionally also a single 

 spicule deviates from the general trend of the rest, sometimes even 

 to the extent of lying athwart them at right angles ; and then also 

 one has an opportunity of examining a spicule separately. In speci- 

 mens where the matrix of the network consists of the phosphatic 

 material called " coprolite," we find that this amber-coloured sub- 



