256 W. J. S0LLAS ON" A NEW LYSSAKINE HEXACTINELLID 



transparent throughout ; it is frequently colourless, but sometimes 

 presents a faint yellowish browm tint either in places or throughout. 

 The surface of the spicule has a rough irregular appearance when 

 examined dry, which disappears, however, on immersion in balsam, 

 being evidently due to a crystalline incrustation (magnesite). When 

 this is removed the sides of the spicules appear as neat continuous 

 lines ; the axial canal is also quite sharply denned, and is sometimes 

 not more enlarged than in recent deciduous spicules, though 

 occasionally it is so wide as to reduce the spicule to a mere shell. In 

 some instances it has been infilled with silica, so as to appear as a 

 cylindrical rod projecting from what remains of the spicule in which 

 it was formed (fig. 8, p. 255). 



The spicules are not excavated by rhombohedral pits like those 

 seen in Hyalostelia Smithi, Young, from the Carboniferous Limestone 

 of Scotland ; but sometimes they are pitted all over with hemispherical 

 cavities such as are seen in deciduous and fossil spicules, and which 

 can be produced artificially in recent spicules by solvent agents. 



With polarized light all the spicules give brilliant colours, and 

 much more vividly than those from the chalk of Trimmingham ; the 

 colours frequently occur in spherical patches, as though crystallization 

 had been set up about various centres within the spicular substance ; 

 one might term it an internal botryoidal structure, though the 

 spicules are never botryoidal externally ; somtimes the polarization- 

 effects show clearly that the spicule is composed of minute prisms 

 radiating outwards from the sides of the axial canal. 



In concluding this brief account of the mineral state of the spicules, 

 I may remark that, though highly crystalline, they are almost if not 

 quite as well preserved as those of Hyalostelia from the Carboniferous 

 formation ; and this is certainly surprising when we consider their 

 greater age and much smaller size. The conditions under which the two 

 sets of spicules were preserved were evidently very different, however ; 

 for Hyalostelia does not appear to occur in association with deposits of 

 chert as is the case with Astroconia. Hyalostelia is possibly preserved 

 to us chiefly by reason of the magnitude of its spicules, Astroconia 

 chiefly by secondary silicification. 



"We now pass on to a description of the forms of the several spicules, 

 which are figured on page 255, all being magnified 40 diameters 

 except figs. 3, 5, and 7, which are magnified 104 times. 



Pig. 1 is the commonest form, occurring in fragments of various 

 lengths, that represented being the longest seen. It is almost exactly 

 cylindrical ; and the axial canal maintains a straight course and even 

 width throughout. Its natural termination has not been seen; but 

 probably it was pointed at both ends, and formed the acerate 

 spicule of the sponge. 



Fig. 2 looks at first sight like the trifid spicule of a Tetractinellid ; 

 but careful examination shows the presence of four rays, of which 

 that marked a is one ; though as here represented it appears 

 merely as a bifurcation of the one in front of it. By examining the 

 other side of the glass slide on which- the spicule is mounted its real 

 nature is readily made out ; moreover I was able to turn an almost 



