310 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
elongated are almost never covered with excrescences, nor otherwise 
grotesquely malformed. 
The measurements of these spicules vary extremely, so that average lengths 
are quite impossible to give. The longest rays do not seem to exceed 
0:26 mm. in length, or 0°012 mm. to 0-014 in thickness. Dwarfed rays, 
however, seem to grow much thicker than the elongated ones as a general 
rule. They frequently measure 0:02 mm. in diameter across the ray itself, 
or, including the excrescences, anything up to 0:03 mm. or even 0:035 mm. 
These measurements, however, are quite probably exceeded in other spicules, 
as there does not seem to be any reason why the growth of these excres- 
cences should be limited to these dimensions. 
Abnormal trizenes also occur in Pachastrella abyssi, Sollas (29). 
(6) Oxea. 
Gi.) The oxea of the main skeleton are long and fairly stout. They are 
pointed at both ends, and the thickest point is in the middle of the spicule 
length, from which there isa gradual tapering towards either end. They 
measure from 2 mm. to 3 mm. in length, and from 0:01 to 0°015 mm. in 
diameter at the thickest part. These spicules form the greater part of the 
spicular bundles and always lie very strictly radially. They form a large 
proportion of those spicules which project from the surface of the sponge. 
(ii.) The tangential cortical oxea are straight, rather slender oxea, which 
do not reach so great a length as do the oxea of the main skeleton. They 
are sharply pointed at both ends and measure 0°8 mm. to 1°4 mm. in length. 
Their thickness does not exceed 0°008 mm. 
B. Microscleres. (Text-fig. 7.) 
(#) he somal microxea lie irregularly scattered throughout the choano- 
some and very occasionally in the cortex. They are 
slender and sometimes almost raphide-like. They measure 
0°15 mm. in length by 0°:002 mm. in diameter. 
5 It is somewhat doubtful whether these spicules should 
SS be described as megascleres or microscleres, but I follow 
a Dendy (11) and Sollas (29), who have previously described 
these spicules as microscleres in other species. 
- (8) The only microscleres which occur in this species 
in addition to the small somal oxea are very small sigmata, 
( which occur in very large numbers throughout the choano- 
some. ‘They possess the characteristic §- or C-shape of 
Fig, 7.— Paratetalla : yP ; s Cc Ie 
cccentrica. Micro. wl the sigmata found among the Tetillidee, and measure 
scleres, X 350. about 0:01 mm. to 0°02 mm. across. Their diameter does 
not exceed 0-001 mm. 
The present species forms the third known species in this somewhat 
