^H. equina.^ Some of the sponges of this species are massive, spherical, and attached by a small base; others are horizontally expanded or cake-shaped; some have a depression in the upper surface and become cup-shaped.
^H. equina^, variety ^cerebriformis^. Massive, circular, cake-shaped, often depressed in the center, producing a cup-shape, attached by broad base; surface broken up by parallel longitudinal ridges having many tufts. Cup-shaped forms predominate, and have a more or less rough surface. This is one of the species known as grass-sponges.
^H. equina^, variety ^meandriformis^, the velvet sponge. The surface of this variety has a protruding flattened cushion of fiber which slightly resembles the convolutions of the brain-coral. Sometimes these cushions are extended into long pencils. The oscula are large and ragged on the edges; the shape is irregular. The average size is seven to eight inches in diameter. (Plate XL.)
^H. equina^, variety ^elastica^ (variety ^agaricina^, Hyatt), the yellow sponge. This is the second grade of commercial sponge, corresponding to the Zimocca sponge of the Mediterranean. It is found growing with the "sheepswool" in a depth of two to twenty feet, and is abundant. It is massive and cake-shaped. The whole surface is a network covered with numerous small, fine cones. (Plate XL.)
The variety ^dura^ is classed with this species, which it resembles in appearance, though it is harder in texture.
^H. canaliculata.^ Massive, frondose; more or less horizontally expanded; bears finger-like processes of varying development on the upper surface.
^H. canaliculata^, variety ^gossypina^, the sheepswool sponge. This is the highest grade of the commercial bath-sponge. It is called "sheepswool" because, perhaps, of its irregular shaggy surface. It is covered with tufts, the larger oscula occupying the intervening depressions.
[Illustration: PLATE XL. Euspongia officinalis, var. adriatica. Hippospongia equina, var. meandriformis. Hippospongia canaliculata, var. gossypina. Hippospongia equina, var. elastica. Euspongia officinalis.]
Sometimes these are very numerous, the whole interior being cavernous; again, the structure is more dense, with fewer large openings and more of the small ones between the tufts; again, the depressions are filled up so that the surface has fewer tufts. When living, the color is shining black. This is the best sponge found on the American coast, and although of coarser texture than the best Mediterranean sponges, it is more durable and quite as elastic. (Plate XL.)