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THE YOFAGE OF H.M.S. CHALLENGER. 
Genus 1. Placospongia, Gray. 
Placospongia, Gray, Proc. Zool. Soc. Lond., pp. 127-129, 1867. 
The only genus of the family. 
Type — Placospongia melobesioides, Gray (p. 271). 
That no little doubt should exist with regard to the systematic position of this 
remarkable family is only natural. Tylostyles do not occur as essential spicules in any 
other Tetractinellid Sponge, while they are characteristic of the monaxonid family 
Suberitidse ; there is something, therefore, especially as trisenes are absent, to be said for 
Carter’s view, according to which they should be placed with the last-named family. 
The tylostyle is not, however, so characteristic and differentiated a spicule as the 
sterraster, hence I prefer to follow Gray in grouping the family with the Geodiidse. 
Suborder III. MIGEOSCLEEOPHOEA, Sollas. 
Tetradina, Sollas (pars), Sci. Proc. Roy. Dabl. Soc., vol. v. p. 177, 1886. 
Mierosderophora, Sollas, Article “Sponges,” Encyclopaedia Britannica, vol. xxii. p. 423, 1887. 
Choristida in which megascleres are absent ; the characteristic microscleres are 
either tetractinose asters, candelabra, or minute trisenes. 
In my preliminary report the Tetradina formed the first suborder of the Choristida, 
and included the Pachastrellidse ; on the removal of this family to the Streptastrosa, 
with which it should evidently be associated, the Tetradina ceases to be a distinctive 
name, but the essential character of the group, as one in which the microscleres have 
not yet given rise to megascleres, is brought into prominence ; hence the change in name 
and definition. 
Family I. P l a c i n i n F. E. Schulze. 
Plakinidse,, F. E. Schulze, Zeitschr. f. wiss. Zool., Bd. xxxv. p. 407, 1880. 
Mierosderophora with tetractinose, triactinose, and diactinose asters, and sometimes 
mono-, di-, or trilophous candelabra. The chamber-system is either eurypylous or 
aphodal ; the mesoderm chiefly collenchymatous. The Sponge is divided into a hypo- 
mere and spongomere. 
Genus 1. Placina, F. E. Schulze. 
Plaldna, F. E. Schulze, Zeitschr. f. wiss. Zool., Bd. xxxiv. p. 448, 1880. 
Incrusting Sponges, with one or more oscular tubes projecting from the free surface. 
Ectosome not differentiated. Chamber-system, eurypylous. Mesoderm scanty, entirely 
