HYALONEMA (HYALONEMA) TYLOSTYLUM. 
231 
It bears spines which are somewhat irregular, strongly divergent, often vertical 
below, and which increase in inclination 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 ix. The lateral rays are conical, pointed or somewhat blunt, very spiny, 
and 45-68 m long. 
The (hypodermal and hypogastral) pentaclines (Plate 69 , fig. 7) have a 
conical blunt proximal ray 0.5-0. 8 mm. long, and 15-40 m 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 unequal, 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 ix. 
The amphioxes (Plate 69 , figs. 11-13) are generally slightly and uniformly 
curved, 0.6-3. 4 mm. long, and 10-30 ^ thick near the middle. A central tyle can 
usually be made out, but it is quite insignificant, as it was not more than 3 ix 
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 /x thicker than the adjacent parts of the spicule, 
and measures 16-52 [x 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 n thick. In the 
small (short) tylostyles it tapers gradually from the tyle to the opposite 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 /x 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 microhexactines (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 /x , generally 
104-140 fx. In most of them all the six rays are nearly straight. In some, one 
