388 



Scientific Proceedings, Royal Dublin Society. 



These observations f urnisli a further strong argument in favour 

 of the originally calcareous nature of the Pharetrones. 



CrijstaUine Structure. — We are now provided with observations 

 sufficient to investigate more closely the crystalline structure of 

 calcareous spicules. 



Commencing with the acerate spicules, we find that in the 

 position in which they usually rest the optic axis lies in them 

 horizontally, inclined about 70° to their morphological axis, and 

 that an axis of maximum elasticity at right angles to the optic 

 axis passes through them vertically. 



Fig. 7. 



Diagram showing the position of a primitive rhombohedron, as it would be seen within an 

 acerate spicule of a calcisponge, the optic axis being placed horizontally, b, polar edge ; 

 d, d, lateral edges of the rhombohedron ; o a, optic axis ; s s, direction of striae^produced 

 by etching: they are parallel to the edges which lie in a zone with d. 



Without spending time in discussing the various positions in 

 which a primitive rhombohedron may be placed in order to satisfy 

 these conditions, let us proceed at once to describe its actual rela- 

 tions to the spicule as deduced from a consideration of the whole 

 of the evidence. A primitive rhombohedron lies within the spicule, 

 so that one lateral solid angle is lowermost, and four parallel edges 

 are inclined at an angle of about 56° with the edge of the spicule 

 as seen through the microscope. These edges are those which give 

 rise to the parallel striae of the etch-figures; four other parallel 

 edges make an angle of 22° with the spicule edge, and the remain- 

 ing four parallel edges similarly make an angle of 46°. It is the 

 last two groups of edges which give rise to the transverse strise of 

 the etch-figures. 



