EEPORT ON THE TETRACTINELLIDA. 
225 
in other cases), and which thence made their way into the choanosome and so to the 
exterior. 
The cortex (PI. XL. fig. 3) measures about O' 79 mm. in thickness, the ectochrotal 
layer containing the sand-grains is about 0'4 to 0'45 mm. thick, the sand-grains are 
angular or subangular and irregular in shape, varying considerably in size from about 
0'16 to 0'4 mm. in length. The pencils of radiatiug cortical oxeas (No. 2) pass between 
adjacent sand-grains, several pencils surrounding each individual grain as though keeping 
it in place. The layer of sterrasters is sharply defined from the preceding, and measures 
about 0'24 mm. in thickness. The usual fibrous layer, about 0'0475 mm. thick, com- 
pletes the cortex on the inner side. The somal megascleres have the usual arrangement 
in radiating sheaves, but they are frequently crossed by small scattered oxeas, similar to 
those of the cortex. The pores (PI. XL. fig. 4) present the usual characters, and vary 
from about 0'025 to 0'05 mm. in diameter, a large oval pore often measuring 0'067 by 
0'028 mm. The canal system is more open than in Cydonium eosaster, the mesodermal 
sarcenchyma not having encroached so far upon the lumen of the canal, hence the 
aphodi are usually short, and indeed occasionally absent, the chambers then becoming 
eurypylous. The chambers are small, about 0'02 mm. in diameter on an average. 
Cydonium eosaster, n. sp. ('PI. XX. fig. 22 ; PI. XXL figs. 15-29). 
Sponge (PI. XXL fig. 15). — Spherical, free, oscules not distinguishable from the pores 
which occur in sieves overlying the chones ; surface smooth, even, with a few small, 
rounded, solid warts or tubercles ; cortex thin, the minute spherasters which underlie the 
outer epithelium are succeeded by a layer of larger spherasters of different form (PL 
XXL fig. 23) ; the rest of the cortex is formed by the sterrastral layer. 
Spicules. — I. Megascleres. 1. Somal oxea (PL XXL fig. 16), fusiform, usually 
sharply pointed ; 2'856 by 0'032 mm. 
2. Cortical oxea, 0'25 to 0'3 by 0'0035 mm. 
3. Dichotrisene (PL XXL fig. 17), rhabdome conical, much attenuated proximally to 
a very sharp point, or rounded off near the end ; protocladi projecting outwards and only 
slightly forwards, deuterocladi directed outwards and slightly backwards. Rhabdome 3 '5 7 
by 0'047 mm., protocladi about O'll, deuterocladi about 0'21 mm. long. 
4. Protrisene (PL XXL fig. 20), rhabdome slender, tapering towards the cladome ; 
cladi directed forwards and only slightly outwards. Rhabdome 5'0 by 0'026 mm. when 
widest, 0‘019 below the cladal origin; cladi 0'19 mm. long, chord 0'19 mm. 
5. Anatrisene (PL XXL figs. 18, 19), rhabdome long, slender, conical, attenuated 
proximally, either very sharply pointed, or strongylate ; cladi extending chiefly outwards, 
very slightly backwards. Rhabdome 8’2'1 by 0'029 mm., cladi 0'095 mm. long, chord 
0T9 mm., sagitta 0'048 mm. 
(ZOOL. CHALL. EXP. PART LXIII. — 1887.) 
Err 29 
