I. Gemmule provided with a thick, apparently granular pneumatic coat in which the gemmule-spicules are arranged tangentially or vertically. (Subgenus _Euspongilla_, p. 69.) A. No foraminal tubule. _a._ Sponge bright green, soft and compressiblewhen fresh, very fragile dry _lacustris_, p. 69. _a'._ Sponge white or grey, hard both fresh and dry _alba_, p. 76. B. A foraminal tubule present. _b._ Skeleton-spicules smooth. beta. Gemmules free; gemmule-spicules arranged tangentially and horizontally _proliferens_, p. 72. beta'. Gemmules free; gemmule-spicules arranged vertically or nearly so in a single series _hemephydatia_, p. 82. beta''. Gemmules firmly fixed to the support of the sponge; gemmule-spicules almost vertical, irregularly arranged, as a rule in more than one series _travancorica_, p. 81. _b'._ Skeleton-spicules spiny or irregular in outline. beta'''. Gemmule-spicules tangential and horizontal, without rudimentary rotules _cinerea_, p. 79. beta''''. Gemmule-spicules vertical or nearly so, often with rudimentary rotules at the tips _crateriformis_, p. 83. II. Gemmules surrounded in several layers by distinct polygonal air-spaces with chitinous walls. (Subgenus _Eunapius_, p. 86.) A. Gemmules single. Skeleton- and gemmule-spicules smooth, pointed, not very stout _carteri_, p. 87. B. Gemmules bound together in pairs. Skeleton friable; skeleton-spicules slender _gemina_, nov., p. 97. C. Gemmules bound together in free groups of more than two or forming a "pavement-layer" at the base of the sponge. _c._ Skeleton friable; skeleton-spicules slender _fragilis_, p. 95. _c'._ Skeleton very hard and resistant; skeleton-spicules stout _crassissima_, p. 98. III. Gemmules without or with irregular pneumatic coat, covered by a chitinous membrane or membranes in which the gemmule-spicules lie parallel to the surface. (Subgenus _Stratospongilla_, p. 100.) A. Skeleton spicules spiny or irregular in outline. _a._ Skeleton-spicules blunt; gemmules covered by a single chitinous membrane _indica_, p. 100. _a'._ Skeleton-spicules sharp; gemmules covered by two chitinous membranes _bombayensis_, p. 102.