HYALONEMA (THALLONEMA) GEMINATUM. 391 



tine. The distal ray is straight and 178-270 n long, most frequently about 

 250 IX. It usually ends in a rather stout and sharp-pointed terminal cone (Plate 

 103, figs. 59-62). Exceptionally the tip is rounded (Plate 103, fig. 58). The 

 basal thickness of the ray is 7-11 yu. Apart from small parts of it at the basal 

 and distal ends, the ray is quite densely spined. The spines are for the most 

 part nearly straight. Those arising from the middle-part of the ray are of 

 ccinsiderable size, up to 30 ii and more long. The maximum thickness of the 

 distal ray, together with the spines, is 32-46 tx. The lateral rays are conical, 

 blunt-pointed, spined, and 28-43 ^ long. 



The small, probably canalar pinules are pentactine. The distal ray is 

 straight, usually 90-110 m long, and 4-6 n thick at the base. Its base and its 

 sharp-pointed distal end-part are smooth; its middle-part bears very large, 

 strongly divergent, sparse spines, which are curved, concave towards the tip of 

 the ray. The largest spines are found in the proximal part of the spine-bearing 

 region. The lateral rays are conical and 32-60 fi long, generally 40-50 fx. 



The hypodermal and hypogastral penlactines have smooth, straight, and 

 \ery blunt, conical rays. The proximal ray is 0.2-1 mm. long, and 7-22 ^ 

 thick at the base. The lateral rays are 160-450 ix long. 



The hexadine megascleres are generally 0.8-1.8 mm. in diameter and have 

 smooth, blunt, conical rays 15-45 ^ thick at the base. 



The superficial and choanosomal amphioxes are usually fairly straight, 

 rarely markedly curved, and 0.4-2.6 mm. long. The shorter ones, that is those 

 under 1 mm. in length, are distinctly centrotyle with a tyle 1-4 fx more than 

 the adjacent parts of the spicule in transverse diameter. The medium ones, 

 that is those 1-1.5 mm. in length, have only a very insignificant tyle, not more 

 than 1.5 M thicker than the adjacent parts of the spicule. In the large ones, 

 that is those over 1.5 mm. in length, there is hardly a trace of a central tyle. 



The microhexadines (Plate 103, figs. 39-48) are 37-120, usually 53-100 m 

 in diameter (maximum diameter). Most of them have equal rays; in some 

 two opposite rays are longer than the other four. The rays are 1.8-2.2 /^ thick 

 at the base, conical, and finely pointed. The proximal part, about the half of 

 the ray, is straight, the distal part curved. This curvatiu-e is either uniform, 

 or, more frequently, at the point where the proximal straight part passes into 

 the distal curved part greater than elsewhere. The total curvature is such that 

 the directions (tangents) of the proximal and distal end-parts of the ray usually 

 enclose an angle of about 120°. The rays bear very numerous, exceedingly 

 minute spines, which give them the appearance of being rough. 



