108 LANUGONYCHIA FLABELLUM. 



It has been stated above that some of the fully developed rays are more 

 cylindroconic and less sharply pointed than the great majority of rays. Such 

 blunt rays have only been observed in the hexactines and pentactines, and it is 

 always the distal protruding ray of these spicules which exhibits this pecuUarity. 

 This differentiation is interesting, since it would, if further developed and associ- 

 ated with an increase in the size of the spines, convert these superficial hexactines 

 and pentactines into pinules. 



The onychhexasters (Plate 13, figs. 6b, 15b, 27) measure 86-135 n in total 

 diameter. Their main-rays, which are regularly arranged and enclose angles 

 of 90° with their neighbours, are 5-8 m long, thickened at both ends, and, in the 

 middle, where thinnest, 2-3 n in transverse diameter. From the end of each 

 main-ray several, most frequently four, branch-rays arise. These are con- 

 siderably curved, convex to the centre of the spicule, at the base, but soon 

 straighten out, often, however, exhibiting slight bends farther on. They are 

 48-60 II long, 1.3-2 n thick at the base, and gradually attenuated to 0.3-0.6 n 

 at the end. They bear along their length sparse, minute, backwardly directed 

 spines, and on their ends two to four, most frequently three, slender terminal 

 spines, 3-5 n long. These usually enclose angles of 90-120° with the end-ray, 

 and are curved, concave towards the centre of the spicule, or nearly straight. 

 They generally arise from the same point, quite terminally, and form a verticil. 

 Sometimes, however, one is situated a little below the end of the end-ray. 



All the discohexasters (Plate 13, figs. 1-4, 5a, 6a, 15a, 17-26) have very 

 much the same shape, but they differ quite considerably in regard to their size 

 and the number of their end-rays. I measured 22 of them and found that 



was under 80 



2 were 81-100 

 6 " 101-120 



3 " 121-140 

 was 141-160 



2 were 161-180 

 6 " 181-200 



3 " 201-220 

 was over 221 ju 



in total diameter. 



This gives the following frequency-curve (Fig. 3). 



From this remarkably regular double curve I conclude that two kinds of 

 discohexasters are to be distinguished, a large kind over 150 n in diameter and a 

 small kind under 150 /i. 



