52 



the curve (fig. 4), then one or both ends of the simple spicule 

 are bifurcated ; another spicule has three arms radiating from 

 a common centre as shown in the central spicule ot fig. 3 ; 

 other spicules have a straight or curved main stem from 

 which one to four branches are given off at various angles 

 and in different directions, each branch, as well as the stem 

 itself having expanded terminations (figs. 5 , 7 , 9) , in other 

 spicules one end of the main stem is not expanded but ter- 

 minates in an obtuse point (figs. 6, 8); in short, there is an 

 endless multiplicity of form of which only the more striking 

 varieties are indicated in the figures. At the terminations of 

 the stems and branches of these spicules, there is a more or 

 less elongated expansion at right angles to the direction of 

 the branch, and this is hollowed out lengthways in such a 

 manner that the depression exactly fits the convex surface 

 of the arms of other spicules and tightly grasps them so as 

 to form a strong complicated network with irregular openings. 

 A few specimens occur yet showing the manner in which the 

 spicules of the mesh were- united together (figs. I, 3) but 

 these examples are very rare in comparison with the number 

 of isolated spicules. Nowithstanding the great variety of form, 

 the spicules are very uniform in thickness , and one may com- 

 pare the skeleton to a piece of complicated wire netting made 

 up of innumerable fragments , cut and twisted into different 

 forms but all from the same strand of wire and fastened to- 

 gether in a similar manner. The surface of the spicules ap- 

 pears to have been originally smooth, though now rough and 

 pitted, and the delicate edges of the expanded extremities have 

 in most instances lost their evenness of outline. It is per- 

 haps owing to the erosion of these extremities that the spi- 

 cules lost their hold and became detached from each other. 

 In one solitary instance only have I noticed a canal in the 

 interior of a spicule. 



The type, and up to the present, the only described spe- 

 cies of the Genus Lyidium is a fragment of sponge dredged 



