REPOUT ON THE TETRACTINELLIDA. 
cxlix 
sphinctrate apertures (Fig. XIV., c) ; in the adult the cloaca may persist or become con- 
verted into a shallow depression, which is usually surrounded by a more or less sharply 
defined margin. 
Type — Geodia gibberosa, Lamarck (p. 244). 
Genus 6. Synops, Vosmaer, 
Synops, \^osmaer, NiederL Arcliiv f. Zool., Suppl. L p. 50, 1882. 
The poriferous and oscular surfaces are distinct. Oscules the single openings of 
excurrent chones; incurrent chones with cribriporal roofs (Fig. XIV., d) 
Type — Synops pyriformis, Vosmaer (p. 266). 
Genus 7. Isops, Sollas. 
Isops, SoUas, Ann. and Mag. Nat. Hist., ser. 5, vol. v. p. 396, 1880. 
Oscules and pores similar, both the simple apertures of similar uniporal chones 
(Fig. XIV, e). 
Type — Isops phlegrsei, Sollas (p. 267). 
Subdivision of the Genera of the Geodina. — The species of Geodia and Cydonium 
are so numerous that, for purposes of convenience, they are artificially grouped in 
sections determined by the number of different kinds of spicules they possess ; these 
sections are as follows ; — 
Section 1. Pantseosa, species with both somal and cortical oxeas, and anatrisenes 
or protrisenes or both, in addition to orthotrieenes or dichotrisenes. 
Section 2. Dirabdosa, species with both cortical and somal oxeas, but without either 
anatrisenes or protrisenes. 
Section 3. Ditrisena, species without cortical oxeas, but with anatrisenes or protrisenes 
or both, in addition to orthotrisenes or dichotrisenes. 
Section 4. Monotrisena, species with but one form of oxea and but one form of 
trisene, which is never either an anatrisene or a protrisene. 
Family II. Placospongida:, Gray. 
Placospongiadx, Gray, Proc. Zool. Soc. Lond., p. 549, 1867. 
Placospongina, Carter, Ann. and Mag. Nat. Hist., ser. 5, vol. vi. p. 55, 1880. 
Placospongidx, Sollas, Encyclopaedia Britannica, vol. xxii. p. 423, 1887. 
Sterrastrosa not possessing trisene spicules ; the only megascleres are tylostyles. 
The sterrastral layer of the cortex is subdivided into irregular or polygonal plates, 
which are united together by fibrous tissue. 
