160 
HYALONEMA (HYALONEMA) OBTUSUM. 
longest is sometimes more than twice as long as the shortest. The greatest 
difference in length between the lateral rays of the same hypodermal pentactine 
observed was 310 /x. The lateral rays are straight, conic, and blunt. They are, 
in the large hypodermal pentactines of var. gracilis, 240-730 /x long and 32-60 /x 
thick at the base. In the small ones they measure 135-440 n in length. The 
small hypodermal pentactines accordingly have, relative to the proximal ray, 
considerably longer lateral rays than the large ones. At the end the lateral rays 
are usually from one fifth to one third as thick as at the base, and here measure 
5-22 n in transverse diameter. The hypodermal pentactines of var. robusta 
have a proximal ray 0.47-1.3 mm. long, and 40-80 m thick at the base. The 
lateral rays are on the whole attenuated towards the distal end less than in the 
hypodermal pentactines of var. gracilis. They are, when not reduced, 250-750 /x 
long and, at the base, about as thick as the proximal ray. 
The end-parts of the lateral rays of these spicules exhibit remarkable irregu- 
larities of external shape and internal structure. These irregularities are the 
more conspicuous the thicker (the more blunt) the rays. Such an irregular 
lateral ray-end of a hypodermal pentactine of var. gracilis is represented (Plate 
33 , fig. 17). A rudiment of a branch-ray, arising a short distance from the tip 
of the main-ray, and a marked irregularity in the axial thread and the stratifi- 
cation of the siliceous body of the latter are noticeable in this spicule. I am 
inclined to ascribe these irregularities to the influence of the obstacles — other 
spicules — met by these rays during their longitudinal growth. The cells 
building the tips of these rays were forced to act in an abnormal manner; being 
prevented by other spicules from adding to the length of the axial thread and from 
depositing silica around it in a normal and regular manner, they produced the 
irregular structures observed. The obstacles (other spicules) which thus cause 
these irregularities are probably the stout proximal rays of adjacent hypodermal 
pentactines. 
The hypogastral pentactines of the gastral cone of var. gracilis have straight 
proximal apical rays, usually 240-400 /x long, and about 12-20 n thick at the 
base. The lateral rays are slightly inclined towards the proximal ray, straight, 
and generally 180-250 ^ long. In the specimen of var. robusta the parts contain- 
ing these spicules appear to have been lost. 
The hexactine megascleres of the distal part of the choanosome (Plate 34 , 
figs. 5-18, 19d) are in both varieties very variable in size and have a maximum 
diameter of 350 n~ 2 mm. The rays of the same spicule are often unequal. The 
greatest difference of length observed in them was 400 n. The rays are 130-950 n 
