HYALONEMA (OONEMA) BIANCHORATUM PINULINA. 
313 
also somewhat hour-glass-shaped, and is at its thinnest point 9-17 /x thick. 
Its spines are usually quite strongly divergent, and markedly curved, concave 
towards the ray. They attain their greatest size one half to two thirds of the 
length of the distal ray from the centrum, or still higher up, and here the distal 
ray, together with the spines, attains a maximum thickness of 30-65 m- The 
lateral rays are similar to those of the pinules of the upper part of the sponge, 
above described, but stouter and provided with larger spines. They attain a 
length of 35-80 /x. Of hexactine forms I found (and measured) only two, with 
proximal rays 12 and 20 ^ long respectively. One of these is conical and pointed, 
the other (Plate 82 , fig. 26) cylindrical and terminally rounded. 
The hypodermal pentactines (Plate 83 , figs. 65-67) have very blunt, conical 
rays. The proximal ray is often somewhat curved ; the lateral rays are usually 
straight, occasionally curved in the plane, vertical to the proximal, in which they 
lie. The proximal ray is 0.4-1. 3 mm. long, and 15-70 m thick at the base. The 
lateral rays are 260-610 /x long. Those of the same spicule are usually fairly equal, 
more rarely conspicuously unequal. In a hypodermal pentactine with particu- 
larly unequal laterals the longest is 610 m in length, the shortest only 390 n. 
The hypogastral pentactines (Plate 83 , fig. 68 ) are similar to the hypodermal, 
but much smaller. Their proximal ray is 210-800 /x long, and 12-46 ^ thick 
at the base. Their lateral rays are 150-460 n long. 
The hexactine megascleres (Plate 83 , figs. 63, 64) are 0.35-5.5 mm. in maxi- 
mum diameter. Their rays are straight or slightly and irregularly curved, 
7-120 /x thick at the base, attenuated distally, at first more gradually, then more 
rapidly, and pointed at the end. In all the larger and in many of the smaller 
ones two opposite rays are considerably longer than the other four. Some of 
these spicules are nearly twice as long as broad. 
The dermal, gastral, and choanosomal amphioxes are straight or more or less 
curved, rarely angularly bent, centrotyle, 0.5-2. 8 mm. long, and 7-59 ^ thick 
near the tyle. The tyle is 9-60 m in diameter, that is 1-23 m, usually 2-6 /x, more 
than the adjacent parts of the spicule. The dimensions of the different kinds 
of these spicules (dermal, gastral, and choanosomal) in the two specimens are 
given in the table on page 314. 
The amphioxes of the dermal and gastral membranes are in both specimens 
considerably shorter, stouter, and less curved than those of the choanosomal. 
The gastrals are in both specimens stouter than the dermals. The amphioxes 
of specimen h are on the whole slightly stouter than those of a. This difference 
is particularly well-pronounced in the gastrals. The two limbs of the angularly 
