HYALONEIMA (HYALOXEMA) TYLOSTYLUM. 231 



It bears spines which are somewhat irregular, strongly di^■ergent, often vertical 

 below,, and which increase in inchnation towards the ray distally. The longest 

 spines arise from the thickest part of the ray, a little below the middle of its 

 length. The maximum thickness of the distal ray, together with the spines, 

 is 23-33 Ai. The lateral rays are conical, pointed or somewhat blunt, very spiny, 

 and 45-68 ^ long. 



The (hypodermal and hj'pogastral) pentactines (Plate 69, fig. 7) have a 

 conical blunt proximal ray 0.5-0.8 mm. long, and 15^0 n thick at the base. 

 ' The lateral rays are straight, conical, blunt, usually 0.3-0.5 mm. long, rarely 

 up to 1.4 mm.; in the same spicule they are often uiiequal, and vertical to the 

 proximal ray or inclined towards it. The angle between proximals and laterals 

 is 80-90°. 



The hexactine megascleres (Plate 69, fig. 6) are 0.4-1.3 mm. in diameter. 

 Their rays are conical, straight, and frequently unequal. Occasionally one ray 

 is reduced in length, cylindrical, and terminally rounded. The basal thickness 

 of the rays is 13-37 ju. 



The amphioxes (Plate 69, figs. 11-13) are generally slightly and uniformly 

 curved, 0.6-3.4 mm. long, and 10-30 n thick near the middle. A central tyle can 

 usually be made out, but it is quite insignificant, as it was not more than 3 n 

 thicker than the adjacent parts of the spicule in any of the amphioxes measured. 



The tylostyles (Plate 69, figs. 8-10) are nearly straight, and 0.8-3.1 mm. 

 long. The terminal tyle is 6-22 /i thicker than the adjacent parts of the spicule, 

 and measures 16-52 ix in transverse diameter. It is usually spherical and quite 

 smooth. Sometimes (Plate 69, fig. 10) a short oblique spine arises from it. 

 The shaft ends in a blunt point. Close to the tyle it is 10-30 m thick. In the 

 small (short) tylostyles it tapers gradually from the tyle to the ojiposite blunt- 

 pointed end. In the medium tylostyles it is cylindrical, of nearly uniform thick- 

 ness for the greater part of its length, and tapers towards the blunt-pointed end 

 only in the ultimate third of its length. In the large tylostyles the shaft is 

 spindle-shaped and sometimes 20 ^ thicker in its middle-portion than just below 

 the terminal tyle. 



The fragments of stalk-spicules observed are smooth and, at the point where 

 they emerge from the sponge-body, have a maximum thickness of 0.5 mm. 



The microhexadines (Plate 70, figs. 4, 5a, 7) and their rare pentactine-deri- 

 vates (Plate 70, fig. 5b) are quite regular, the rays of the same spicule being 

 fairly equal in size. The total diameter of these spicules is 75-170 m, generally 

 104-140 fi. In most of them all the six rays are nearly straight. In some, one 



