i6 



feet as in recent spicules. Another feature of the little 

 change which these spicules have suffered, is shown by their 

 being neutral to polarised light, thus proving that the silica 

 still retains its colloidal condition, although it has lost that 

 beautiful clear glassy appearance which characterizes the spic- 

 ules of recent sponges. The state of preservation in which 

 these Coesfeld spicules are found may, however, be regarded 

 as exceptional, in the majority of instances in which detached 

 spicules have been noticed the silica appears to be now in the 

 form of Chalcedony. 



As I have already mentioned, the eroded condition of 

 the larger spicules plainly indicates that none of those much 

 smaller forms which have been termed flesh-spicules , have 

 stood any chance of being preserved, and therefore, for the pur 

 poses of classification, the characters of the skeleton spicules 

 will alone have to be relied upon. Of these I separated 

 some thousands of specimens and they present the most ma- 

 nifold varieties both of form and size. In dimensions they 

 vary between o.iSmm., the diameter of small globo-stellates, 

 and the comparatively great dimensions of 8.77 mm., the length 

 of the longest trifid spicules. The various forms comprise straight 

 and curved cylindrical acerates, conical and fusiform acuates, a 

 great variety of trifid spicules (i. e. those with an elongated shaft 

 and a head of three rays) ; qdadrifid spicules , globates and 

 stello-globates, contorted skeleton spicules of Lithistids and 

 free and combined six-rayed spicules. Besides these different 

 forms of isolated spicules there are a few specimens in which 

 small fragments of the mesh-work of the sponge, showing the 

 spicules naturally attached together, have been preserved. A 

 glance at the figures given in the different plates will better 

 enable an idea to be formed of the great diversity presented 

 by these spicules , mixed up together in the interior oi 

 this single flint, than can be conveyed by a mere verbal de- 

 scription. 



