HYALONEMA (OONEMA) DENSUM. 345 



The table shows that, apart from the few hexactine forms, which appear 

 only partly to conform to the rule, the acanthophores situated farther up are 

 larger and have more slender rays than those situated farther down. The rod- 

 shaped diactine acanthophores are longer than any of the others. Apart from 

 this the size (ray-length) of these spicules is by no means in inverse proportion 

 to the ray-number; the triactines, for instance, are shorter than the tetractines. 

 The thickest rays are met in the tetractines. This applies both to the upper 

 and the lower tetractines. The diactines have a central and often also two ter- 

 minal tyles. The latter sometimes attain remarkably large dimensions. In a 

 spicule of this kind 780 ix long and 15 ^u thick, the two terminal tyles were 

 respectively 50 and 60 m in diameter. The small acanthophores are often spined 

 thi-oughout; in the large ones the spines are confined to the ends of the rays. 

 The spines are low and broad, and attain 10 ix in length and 16 yu in breadth. 

 They are conical and pointed or, more rarely, rounded at the end and dome- 

 shaped. The acanthophores with rounded spines are characteristic of the spe- 

 cies. The rays are straight or curved. Strongly curved rays are met particu- 

 larly among the larger tri- to hexactines situated below. The rays of all the 

 small acanthophores, of all the diactine acanthophores, and of all kinds of 

 acanthophores situated farther up, are usually fairly straight. 



The microhexadines (Plate 94, figs. 37-40) are 60-165 m in diameter, usually 

 95-160 fj.. The rays are equal, conical, finely pointed, 3-6 fi thick at the base, 

 and curved slightly but quite distinctly and quite uniformly throughout their 

 length. They bear spines which are rather sparse, vertical, and sometimes 1 /z 

 high in their proximal part, and which are more numerous, smaller, and directed 

 backwards in their distal part. Toward the ends of the rays the spines decrease 

 in size to such an extent that the end-parts themselves merely appear rough, 

 even under the highest power. 



From a morphological point of view four kinds of amphidiscs can be dis- 

 tinguished: — A, large ones, with broad, short anchors, less than a third of 

 the whole spicule in length, and no protuberance, or only one or two, on the 

 shaft; B, middle-sized ones with long and broad anchors, about half the length 

 of the whole spicule; C, middle-sized ones with short elliptical anchors, about a 

 third of the length of the whole spicule, and a shaft spined throughout; and D, 

 small ones with short U-shaped anchors, less than a third of the whole spicule 

 in length. 



The length frequency-curve of the amphidiscs, shown in Figure 22, exhibits 

 four very distinct elevations separated by deep depressions or gaps. These 



