The genus _Thalassolampe_ is, next to _Actissa_, the most simple of all Radiolaria, but differs from it by the large intracapsular alveoles. By these the central capsule is inflated to an extraordinary size, which in _Thalassolampe maxima_ exceeds that of most other Radiolaria. From the nearly allied _Thalassopila_ it differs by the simple spherical nucleus, from _Physematium_ by the absence of spicula. Of the two species of the genus the first observed _Thalassolampe margarodes_, 1862, is Mediterranean, the second, _Thalassolampe maxima_, 1882, is Indian. 1. _Thalassolampe margarodes_, Haeckel. _Thalassolampe margarodes_, Haeckel, 1862, Monogr. d. Radiol., p. 253, Taf. ii. figs. 4, 5. _Thalassolampe margarodes_, R. Hertwig, 1876, Histologie d. Radiol., p. 68, Taf. iii. figs. 1-5. Spherical body very soft and limpid, somewhat pearl-like opalescent, yellowish or bluish. Central capsule with a very thin structureless membrane, its diameter six to eight times as large as that of the central spherical nucleus. Wall of the vesicular nucleus thick, perforated by fine {17}pore-canals; on its inside often numerous oval nucleoli. In the movable protoplasmic network between the large alveoles a considerable number of large yellowish or orange oil-globules. Extracapsular jelly-envelope very thin, contains small yellow bodies (zooxanthellæ). (Compare the accurate description of this Mediterranean species in my monograph and in Hertwig's work.) In the Canary Islands I found very often a large variety of it, of double and triple the size, distinguished by the delicate orange colour of the intracapsular oil-globules. This may be distinguished as _Thalassolampe aurantiaca_. _Dimensions._--Diameter of the whole jelly-sphere 2 to 4 mm., of the central capsule 2 to 3 mm., of its nucleus 0.2 to 0.4 mm. _Habitat._--Mediterranean, Messina, Haeckel, Hertwig; Canary Islands, Lanzerote, Haeckel; surface. 2. _Thalassolampe maxima_, n. sp. (Pl. 1, fig. 2). Spherical body quite pellucid, like a glass globule, colourless. Central capsule with a moderately thick, but quite transparent, structureless membrane, its diameter ten to twelve times as large as that of the central spherical nucleus. Wall of the vesicular nucleus thick, perforated by fine pore-canals; on its inside numerous small spherical nucleoli. No large oil-globules in the movable protoplasmic network between the large alveoles. Extracapsular jelly-envelope very thin, containing no yellow bodies. This differs from the preceding nearly allied species in the want of the intracapsular oil-globules and of the extracapsular yellow bodies. It possesses the largest central capsule of all known Radiolaria. I found them living and floating in water taken from the surface of the Indian Ocean by a bucket. _Dimensions._--Diameter of the whole jelly-body 12 to 15 mm., of the central capsule 10 to 12 mm., of the nucleus 0.8 to 1.2 mm.