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 ix in transverse diameter. The 

 gastral pinnies of specimen (a) appear to have on the whole more slender distal 

 rays than those of (6). The lateral rays are straight, cylindroconical, and 39- 

 92 jx long, most frequently 45-70 /li. They are spiny. In the gastral pinnies 

 of specimen (a) their spines are usually quite numerous and small, in those of (6) 

 often sparse and very large, 2-4 /i 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 n 

 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 n thick. 



The hypodermal and hypogastral pentactines have a straight proximal ray 

 0.5-1 mm. long, and 10-60 ix 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 megasclcres are 0.5-1.5 mm. in total diameter, 

 and have conical rays 9-27 n 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 /x thick near the middle. The central tyle 

 measures 16-26 n 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 stijle amphiox-derivates are as thick as the amphioxes, but shorter. 

 In these spicules one of the two rays is properly developed, the other reduced 



