EEPORT ON THE TETEACTINELLIDA. 
105 
Spicules. — I. Megascleres. 1. Oxea (PL XL fig. 2), slender, cylindrical, sharply 
pointed, curved or undulating, 2 ‘54 by 0‘016 mm. 
2. Calthrops (PI. XL figs. 3, 4) ; actines conical, sharply pointed, usually simple, 
occasionally dichotomous, sometimes irregularly branched, bent and tubercular (PI. XL 
figs. 15-19) ; a single actine, 0796 by 0'0636 mm. 
II. Microscleres. 3. Centrotylote microxea, slender, sharply pointed, surface 
roughened, 0'083 mm. long. These spicules are precisely similar to the corresponding 
ones of Pcecillastra ; they occur sparingly and locally, and may possibly have been derived 
from a species of Pcecillastra (Pcecillastra incrustans) which incrusts the poriferous 
surface. , 
4. Microstrongyle (PI. XL figs. 11-13), of variable form, sometimes prolately ellip- 
soidal, sometimes cylindrical, with rounded ends, and sometimes centrotylote, then navicula- 
like in outline, surface usually smooth, sometimes roughened ; from 0'014 by 0'0046 mm. 
to O'Ol by 0'0052 mm. This spicule has probably been derived from a diactinose oxyaster. 
5. Spiraster (PI. XL figs. 9, 10), of variable character, sometimes a true spiraster, 
more often a metaster or amphiaster ; axis or spire slender, spines numerous, very slender ; 
O'OllS to 0'0158 mm. long ; a single spine may attain a length of 0‘006 mm. 
Colour. — Cream-yellow. 
Habitat. — Station 135c, Tristan da Cunha, October 17, 1873 ; lat. 37° 25' 30" S., 
long. 12° 28' 30" W.; depth, 110 fathoms ; bottom, hard ground, shells and gravel. 
Florida, 228 fathoms (0. Schmidt, loc. cit., p. 64), and various places in the Gulf of 
Mexico (0. Schmidt, loc. cit., p. 68). Near Cape St. Vincent, 292 fathoms, “Porcupine” 
Expedition, Station 24 (Carter, loc. cit., p. 407). 
Remarks. — Two specimens of this sponge were obtained ; one is entire, the other 
subdivided into six pieces and not quite complete. The perfect specimen is that repre- 
sented in the plate. It is 78 mm. wide, 150 mm. long, and 25 mm. in average thickness. 
The pieces of the other specimen when put together form a mass 200 by 150 mm. in 
length and breadth, and 90 mm. in maximum thickness. 
The oscular surface in both specimens is incrusted by blue sand and various attached 
foreign bodies and organisms. The oscules are about 1 to 1'5 mm. in diameter. The 
poriferous surface, otherwise bare, is overgrown over a considerable area by a species of 
Pcecillastra {Pcecillastra incrustans), the canals of which, passing vertically through its 
plate-like growth, terminate immediately over the poriferous ends of the incurrent canals 
of the Pachastrella (PL X. fig. 15). The ectosome consists of collenchyma which is crowded 
with oval, not very deeply stained, cells, from 0‘016 to 0'02 mm. in diameter; they 
consist of spherical granules, about 0'004 mm. in diameter, which lie close together, con- 
cealing the nucleus if it be present. Each granule-cell lies in a distinct cavity in the 
collenchyme, which consists of a clear, colourless matrix and not very easily discerned 
(zooL. CHALL. EXP. — PART Lxiii. — 1887.) Err 14 
