R. W. H. ROW—REPORT ON THE SPONGES : CALCAREA. 201 
Skeleton arrangement. (P1. 19. fig. 4.) 
A. Dermai cortex. 
The spiculation of the dermal cortex consists entirely of regular to sub- 
regular triradiates (text-fig. 3, c), which usually have the basal ray slightly 
longer than the paired rays. The oral angle is also usually somewhat greater 
than the paired angles, but many of the spicules appear to be absolutely 
regular. The oral rays are usually straight, but may be slightly curved. 
The basal ray is quite straight. All three rays are of the same thickness, 
and taper very gradually till near the end, most of the diminution of thickness 
occurring in the distal third of the ray. They lie in the cortex entirely 
without orientation. 
B. Tubar skeleton. 
(i.) Triradiates (Text-fig. 3, a,).—The tubar skeleton is almost entirely 
composed of prochiacts, intermediate forms and sagittal triradiates, all being 
present in both subdermal and subgastral layers. 
Fig. 3.—Spicules of Grantilla hastifera, all x 60. 
The prochiacts (text-fig. 3,a) are much more numerous in the subgastral 
than in the subdermal layer, where they but occasionally appear. The paired 
rays are equal in length and as a rule about one-third of the length of the 
basal ray ; they are curved very considerably, and taper throughout their 
whole length. The basal ray is quite straight, and likewise tapers through 
its whole length. All the rays are of equal thickness. 
The intermediate forms between prochiact and sagittal triradiate spicules 
also have approximately equal oral rays, and the measurements given below 
for the prochiacts apply equally to them. 
The sagittal triradiates (text-fig. 3, >), however, show a peculiar modi- 
fication of their paired rays, which are of unequal length, one of them being 
very nearly twice as long as the other in a typical specimen, while sometimes 
