116 
THE VOYAGE OF H.M.S. CHALLENGEE. 
and 0'012 mm. along the other; the spherical nucleolus is about 0'0039 mm. in 
diameter. 
Myriastra clavosa (Ridley) (PL XII. figs. 34-43). 
Stelletta clavosa, Eidley, Eep. Zool. Coll. “Alert,” 1881-82, p. 474, pi. xliii. figs. 1, 2. 
Sponge (PI. XII. fig. 34). — Small, spherical, free ; flattened or depressed above, in 
the centre of the depression a single small, circular, or oval oscule, with a thin membran- 
ous margin, which roofs over the cloacal chamber, into which numerous small excurrent 
canals open. Surface even ; pores uniformly distributed in sieve-like areas. 
Flagellated chambers small, on an average 0'02 mm. in diameter. 
Spicules. — I. Megascleres. 1. Oxea (PI. XII. fig. 35) slender, fusiform, straight or 
slightly curved, sharply pointed ; from 2'86 by 0'016 mm. to 3'42 by 0'0316 mm. in 
diameter. 
2. DichotriaBne (PL XII. fig. 36), a slender, conical, usually excessively sharply pointed 
rhabdome ; protocladi projecting forwards and outwards, deuterocladi extending hori- 
zontally. Rhabdome from 2’825 by 0‘035 mm. to 3‘42 by 0‘047 mm. ; protocladi from 
0‘08 to O'll mm. long; deuterocladi from 0'27 to 0’29 mm. long. 
3. Anatrisene (PI. XII. fig. 37), a long, slender, sharply pointed rhabdome, cladi 
conical, regularly recurved, sharply pointed ; front of the cladome usually rounded hemi- 
spherically. Rhabdome 2'32 by 0*002 mm. to 3*02 by 0*024 mm.; cladi from 0*055 to 
0*079 mm. long, chord and sagitta from 0*04 and 0*08 to 0*072 and 0*127 mm. 
4. Cloacal oxea, slender, fusiform, sharply pointed, 0*318 by 0*009 mm. 
II. Microsclere. 5. Chiaster (PL XII. fig. 38), actines tylote, 0*008 to 0*012 mm. 
Colour. — Faint yellowish or greenish-white. 
Habitat. — Station 186, September 8, 1874 ; lat. 10° 30' S., long. 142° 18' E. ; 
depth, 6 fathoms ; bottom, coral mud. Dredged. 
Station 208, January 17, 1875 ; lat. 11° 37' N., long. 123° 31' E. ; depth, 18 fathoms: 
bottom, blue mud. Trawled. 
Also (Ridley), Prince of Wales Channel, and West Island, Torres Straits, 7, to 9 
fathoms ; bottom, sand and coral ; and Arafura. Sea, off north-west coast of Australia, 
32 to 36 fathoms ; bottom, sand, mud, and shells. 
Remarks . — The collection contains a series of specimens all remarkably alike in form, 
structure, and the characters of their spicules ; and it has been a task of great difficulty, 
attended with considerable uncertainty, to define the exact limits of the species. Ridley 
states that the spicules of the numerous specimens which he examined varied but 
slightly in dimensions ; of those which I have regarded as belonging to Myriastra 
clavosa, this, as will be seen from the appended table, can hardly be said ; they differ in 
