74 



instances the tube apparently commences in the centre of the 

 broken end of a spicule, where the canal in the perfect example 

 would be situated and from thence it extends forwards in the 

 substance of the spicule without any regularity , occasionally 

 keeping near the centre or wandering towards different parts 

 of the circumference, without however penetrating the outer 

 wall of the spicule. The interior is apparently filled with a 

 transparent material of a light greenish tint, giving no colour 

 under the polariscope and apparently destitute of any granular 

 particles. The longest of the tubes noticed is 1,8 mm. and 

 the width varies in different specimens between 0,033 nim - 

 and 0,067 mm - 



The mineral character of the spicules in which these bor- 

 ings occur is precisely similar to the normal ones of this 

 material, that is to say, it is how of chalcedonic silica and 

 there seems no reason to doubt that the chalcedonic silica 

 has been produced from the conversion of the original amor- 

 phous silica of the sponge spicule , so that these perforated 

 tubes whether made previous or after the change, have been 

 constructed in siliceous material. The peculiar form of the 

 perforations show that they have been produced by the action 

 of some living organism, of which no other traces are left. 

 These tubes are larger than those which have been described 

 by Prof. Duncan in the substance of Silurian and Tertiary 

 corals, and attributed by him to the boring influence of a 

 parasitic alga, PalacacJiyla perforans (Quart. Jour. Geo. Soc. 

 Vol. XXXII, p. 205) neither are they, branched or filled with 

 the dark granules which appear associated with the borings 

 in the corals. There is further the peculiarity that they have 

 been made in siliceous structures. 



