224 HYALONEMA (HYALONEMA) TENUIFUSUM. 
largest spines are usually those arising some distance below the middle of the 
length of the ray. Proximally they become shorter but remain nearly as thick; 
distally they become both shorter and more slender. The spines on the distal 
part of the ray are usually very small, often quite rudimentary. At the point 
of maximum thickness, which is generally situated below the middle of its length, 
the distal ray is, together with the spines, 10-46 y in transverse diameter. The 
gastral pinules of specimen (a) appear to have on the whole more slender distal 
rays than those of (b). The lateral rays are straight, ecylindroconical, and 39— 
92 » long, most frequently 45-70 u. They are spiny. In the gastral pinules 
of specimen (a) their spines are usually quite numerous and small, in those of (b) 
often sparse and very large, 2-4 4 long. The proximal ray of the hexactine 
forms is similar to the laterals but shorter. 
The canalar pinules (Plate 68, fig. 4) are very variable in appearance and 
form a series one end of which is represented by pinules similar to the gastrals, 
the other by pentactines the apical ray of which is only slightly longer and 
bears only slightly larger spines than the laterals. The distal ray is 68-120 u 
long, and 3.5-7 » thick at the base. It bears a few obliquely ascending, nearly 
straight spines, which attain a considerable size in the larger pinules of this kind. 
At the point of maximum thickness, which usually lies at or below the middle of 
its length, the distal ray, together with the spines, is 8-28 u thick. 
The hypodermal and hypogastral pentactines have a straight proximal ray 
0.5-1 mm. long, and 10-60 » thick at the base. The lateral rays are straight, 
usually inclined more or less towards the proximal ray, and 0.2-1.1 mm. long. 
I have often noticed a great inequality in the length of the lateral rays. In some 
of these spicules the longest lateral is nearly twice as long as the shortest. 
The choanosomal hexactine megascleres are 0.5-1.5 mm. in total diameter, 
and have conical rays 9-27 » thick at the base. Besides the more or less intact 
hexactines from which these measurements were taken, fragments of such 
spicules were observed which indicate that hexactine megascleres also occur of 
dimensions considerably exceeding those given above. 
The centrotyle amphiox megascleres are more or less curved, 0.9-3.4 mm. 
long, usually 1-2.4 mm., and 7-13 u thick near the middle. The central tyle 
measures 16-26 » in diameter, and is 1.2—2.9 times as thick as the adjacent parts 
of the spicule. The thin amphioxes have a relatively larger central tyle than 
the stout ones. 
The style amphiox-derivates are as thick as the amphioxes, but shorter. 
In these spicules one of the two rays is properly developed, the other reduced 
