CAULOPHACUS SCHULZEI. 
53 
is probably due to the potential energy of the silicoblasts building the short rays 
being partly diverted from their normal use of forming long rays and converted 
into the work of producing spines. The longitudinal reduction of the rare, 
smooth, terminally thickened, shortened rays is obviously of a different nature, 
the potential energy of the silicoblasts being in this case diverted in another 
direction. The difference of these two kinds of reduction is probably attribut- 
able to a difference in the cause of the reduction. 
There is no perceptible difference in the dimensions of the hexactine choano- 
somal megascleres of the small (B, 30 mm.-disc) and large (D, 60 mm.-disc) 
specimen. 
Wilson ( loc . cit., p. 44) found the hexactine rays 0.7-1. 2 mm. long, and 
28-48 n thick at the base. He occasionally observed hexactines with spines 
on all rays, but does not mention the forms with spines on the reduced ray only. 
In his figure (loc. cit., Plate 5, fig. 10) all the hexactines are drawn with stout, 
straight, and equal rays. My photographs (Plate 7, figs. 20-31) show that 
in the material examined by me their appearance is different. Since, however, 
the figure (Plate 5, fig. 10) of Wilson is a general view of a section, I believe 
myself justified in assuming that this difference is not real but merely due to the 
hexactines in the figure cited having been drawn schematically. 
The rare pentactine hexactine derivates (Plate 8, figs. 23, 24) are, apart 
from the suppression of one of the rays, similar to the hexactines. Some of 
them bear spines on all the rays. These pentactines are 1. 1-3.1 mm. in 
diameter. The longest of their usually unequal rays, which may be the un- 
paired apical one or another, is 0.8-1. 6 nun. long, their shortest ray 0.4-1. 4 mm. 
The rays are 30-55 m thick at the base. 
The hypodermal and hypogastral pentactines (Plate 8, figs. 1-7, 12-22, 25-27) 
are very similar. Their lateral rays are either all properly developed and 
fairly equal (Plate 8, figs. 19-21, 25-27), or one, two, or three of them are more 
or less reduced in length, shorter than the others (other), and also, if more than 
one, unequal in length among themselves (Plate 8, figs. 12-14, 18, 22). The 
properly developed lateral rays are straight or slightly curved, conic, and very 
blunt. Their proximal part is either smooth or it bears a larger or smaller num- 
ber of spines. Farther on, and up to a short distance from the end, they are 
nearly always smooth. The end itself is either also smooth or densely covered 
with small spines. The proximal spines extend, when present, from over a 
quarter to nearly a half of the length of the ray. They are low, broad, pointed, 
and conic. In regard to their number the lateral rays, even of the same spicule, 
