no 
THE YOYAGE OF H.M.S. CHALLENGER. 
Pachastrella monilifer, 0. Schmidt. 
PacJiastrella monilifer, 0. Schmidt, Spong. Algiers, p. 15, pi. iii. fig. 7, 1868. 
Sponge. — An irregular nodular fragment with a rough surface ; oscules several, from 
0’5 to I'O mm. in diameter. 
Spicules. — I. Megascleres. 1. Oxea, fusiform, almost cylindrical, slender; broken 
specimens measure 0‘03 mm. in diameter and up to 1'75 mm. in length, but when perfect 
they may have been as much as twice as long. 2. Calthrops ; the young forms, up to a 
stage in which the actines measure 0'2 mm. in length, are regular and isoactinate, but 
when fully grown they resemble a trisene in the fact that three of the actines are regularly 
curved forwards like the cladi of a large protrisene, while the fourth is straight and longer 
than the others, resembling the rhabdome of a trisene ; the spicule then presents a 
remarkable similarity to the protrisene of Stryphnus rudis (vide postea). The curved 
actines attain a length of 0’835 mm., the straight actine measures 1’03 by 0‘095 mm. 
In an example in which the curved actines are 0'8 mm. long the chord measures 
0*825 mm. 
II. Microscleres. 3. Microxea, fusiform, curved, 0*35 mm. in length. 4. Micro- 
strongyle, centrotylote, 0*02 by 0*004 mm. 5. Ampliiaster, the axis continued into 
a spine beyond the whorls at each end, spines and axis remarkably slender, total length 
0*0118 mm. 
Habitat. — Coast of Algiers. 
Remarhs. — This species is the type of the genus Pachastrella as instituted by 
Schmidt. I at first provisionally separated Pachastrella ahyssi from it as the type of a 
new genus, Picraster, on the assumption that Schmidt’s description could be depended 
upon. Subsequently I received through the kindness of Professor Perrier a fragment 
taken from Schmidt’s type, and to my surprise I find that it is scarcely specifically dif- 
ferent from Pachastrella ahyssi. Neither in this specimen nor in a type slide of spicules 
presented by Schmidt to the British Museum are any of the concentrically striated 
umbilicated discs described and figured by Schmidt as occurring in Pachastrella monilifer 
to be found, while the amphiaster, which Schmidt may be supposed to have regarded as 
absent, for he does not mention it, is abundant enough in the specimen I received from 
Professor Perrier, though not in Schmidt’s preparation in the British Museum collection. 
The microxeas are sometimes not only bent in the middle but slightly reflexed at each 
end, in a manner which suggests the idea that the toxa of Dercitus may have been 
derived from similar spicules. 
