112 
THE VOYAGE OF H.M.S. CHALLENGER. 
Genus (?). 
Pachastrella lithistina, 0. Schmidt, Spong. Meerb. Mexico, p. 68, pi. ix. figs. 3, 4, 1880. 
Sponge. — In plates 15 to 20 mm. thick, the upper surface produced into many flat 
humps, each bearing a single oscule, 0'5 mm. in diameter ; the under surface with similar 
oscules or pores. These openings lead into a labyrinth below the outer layer of the 
cortex, penetrating the whole thickness of the plate. 
Spicules. — I. Megasclere. 1. Calihrops. 
II. Microscleres. 2. Microstrongyle, growing out in irregular processes, finely 
tuberculated, 0'008 mm. long. 3. Asters, 0’008 to O'Ol mm. in diameter. 
Species originally described as Pachastrellidje which are here referred to 
OTHER Families. 
PoAihastrella intexta, Caxtei = Triptolemus intextus (see p. 102). 
Pachastrella parasiticus, Odxtex = Triptolemus parasiticus (see p. 102). 
Pachastrella amygdaloides, Gaxtex = P(jecillastra amygdaloides (see p. 99). 
Pachastrella connectens, var., 0. Sch. = Characella agassizi (see p. 101). 
Demus II. Euastrosa. 
Astrophora in which euasters are always present, but never spirasters nor sterrasters. 
Trisenes are present but not calthrops. 
Family I. Stellettida;. 
Euastrosa in which the megascleres are oxeas, and orthotrisenes, or plagiotrisenes, or 
dichotrisenes and frequently in addition anatrisenes. The chamber system is aphodal 
and the mesoderm of the choanosome sarcenchymatous. , 
Subfamily 1. Homasterina. 
Stellettidse, which never possess more than one form of aster. 
Genus 1. Myriastra, Sollas. 
Sponge small ; oscules distinct ; pores in sieves, leading into subdermal cavities. 
Ectosome thin, collenchymatous, excavated by widely extending subdermal cavities. 
The single form of microsclere is a chiaster. 
