ERYLUS SOI.LASII. 277 



DIMENSIONS OF RHABD MEGASCLERES. 





Race 



all forms 



I 



Form 



II 



III 



A 



B 



C 



D 



limits 



general aver- 

 age of tlie 

 largest tliree 



Length 



of nil file spie- 

 ules measured, 

 /( 



425- 

 ()5() 



530- 

 730 



470- 

 760 



530- 

 880 



450- 

 760 



720- 

 9S0 



425- 

 980 





a\ erase of the 

 three longest, /< 



630 



710 



747 



853 



743 



913 





766 



Tliickness 



of all the spic- 

 ules measured. 



8-13 



S-14 



10-24 



10-22 



9-20 



12-22 



8-24 





avei'age of the 

 three thickest,// 



12 



13 



22 



20 



19 



21 





18 



Besides these greatly preponderating isoactine forms, some anisoactine ones, 

 with one actine shortened and rounded at the end (Plate 1, figs. 39b, 41b), are 

 met with. In some of the microrhabds of race I, form A, and race II this 

 reduction has gone so far that one actine is absent altogether. Such spicules 

 appear as styles (tylostyles). Also branched microrhabds, composed of more 

 than two actines, have occasionally been met with. These are most frequent 

 in race I, form B. Most of them are tetractine and appear to have been pro- 

 duced by an early concrescence of two simple microrhabds lying crosswise. 

 Two opposite rays of such spicules usually form a microrhabd of similar dimen- 

 sions to the ordinary ones. The microrhabd represented by the two other 

 rays is usually considerably shorter. Rarely the two microrhabds presuma- 

 bly composing these spicules are equal in length. Their axis generally encloses 

 small angles, 30° or less; rarely these angles are greater; forms with axis crossing 

 at right angles are exceedingly rare. Sometimes the one microrhabd is attached 

 to the other by its end; such spicules appear as triactines. The tyle is usually 

 a simple spherical thickening. In the isoactine forms it occupies the centre, in 

 the anisoactine forms it lies nearer to one end than to the other, and in the 

 microrhabds with one actine quite suppressed, it is situated terminally. In a 

 few of the microrhabds of race I, form D, the tyle is irregular and appears as a 

 cluster of rounded protuberances. 



