312 ERYLUS CALKT'LATUS. 



are centrotyle amphistrongyles, usually more or less attenuated towards the 

 rounded ends. They are generally slightly curved, the curvature being uniform 

 or, rarely, greater near the ends than in the middle. Most of these spicules 

 are isoactine, anisoactine forms with one actine reduced in length being met 

 with only exceptionally. The microrhabds are 39-52 /t long and, near the middle, 

 close to the tyle, 3-5 fx thick, the average measurements of the three longest 

 and thickest being 51 by 4.8 /t. The tyle is 0.3-1 /( more in transverse diameter 

 than the adjacent parts of the spicule. 



Besides these regular microrhabds, branched microrhabd-derivates have 

 been observed in small numbers. In these spicules two or, rarely, more, short, 

 terminally rounded or, exceptionally, pointed branch-rays arise from a point 

 a little below one end of the spicule. In one of these spicules such branch-ra)'s 

 were observed at both ends. When there are two branch-rays they usually 

 stand opposite each other in a straight line which intersects the axis of the 

 spicule at an angle of 40 to 60°. 



The triaenes (Plate 5, figs. 6-10; Plate 6, fig. 23) are orthotriaenes. Their 

 rhabdome is straight, conical, 200-300 n long, and, at the cladomal end, 13-20 // 

 thick. The clades are 125-220 // long, usually simple, and slightly and irregu- 

 larly curved. Rarely one clade bears a short branch-ray. The clades enclose 

 angles of 87-98° with the rhabdome. The breadth of the cladome is 210-380 ft. 



The acanthtylasters (Plate 6, figs. 4b, 5b, 6-13, 29; Plate 7, figs. 74, 77b; 

 Plate 8, figs. 2-12) usually have from four to eight, rarely only two or three, 

 concentric and regularly or, more rarely, irregularly distributed rays. The 

 rays are equal or one or more of them reduced in length. The properly 

 developed rays are, at the base, 1.5-4.5 fi thick and taper towards the distal end, 

 which is usually crowned by an acanthtyle, rarely simply rounded off. The 

 acanthtyle or rounded end is 1-5 /£ in transverse diameter. The basal part of 

 the raj's is either quite smooth or bears a few spines; from their central and 

 distal parts a good many spines arise. The acanthtyle (rounded end) is densely 

 covered with spines. The spines are usuallj^ conical, sharp pointed, and of con- 

 siderable size. The largest one observed was 1.5 /z long. The spines arising 

 from the acanthtyle (rounded end) are generally very markedly recurved and in 

 their position resemble short flexible hair combed down in all directions from 

 the apex of the acanthtyle (rounded end). Those arising from the rays farther 

 down are either vertical or directed obliquely outward or inward. The total 

 diameter of the acanthtjiasters is 17-50 fx. This dimension and also the size 

 of the rays and acanthtjdes are, on the whole, in inverse proportion to the ray- 



