The genus _Thalassicolla_ was proposed by Huxley in 1851, for a certain number of different voluminous jelly-like Radiolaria, which he had observed living during his voyage in the "Rattlesnake" through the tropical seas, and of which he gives an excellent description--the first accurate observations on living Radiolaria. Johannes Müller afterwards removed from this genus the social genera _Sphærozoum_ and _Collosphæra_ (formerly _Thalassicolla punctata_), and retained as type of the genus _Thalassicolla nucleata_. In 1862 in my Monograph I added two other species, _Thalassicolla pelagica_ and _Thalassicolla zanclea_, and later (1870) _Thalassicolla sanguinolenta_. Now I think it better to separate the last two species as a new genus, _Thalassophysa_, characterised by the papillate or branched nucleus, and to retain in _Thalassicolla_ only those forms with simple spherical nucleus. For both genera the extracapsular, voluminous, spherical calymma or jelly-envelope, with numerous large alveoles, is characteristic. The membrane of the central capsule in _Thalassicolla_ is now structureless (subgenus _Thalassicollarium_, with three species), now characterised by a peculiar structure, prominent ridges on the inside of the membrane, which form a network with polygonal plates, resembling an epithelium (Pl. 1, fig. 5_b_; subgenus _Thalassicollidium_, with four species). Of the seven species here described, two are cosmopolitan, widely distributed, and common; one is Mediterranean, one Atlantic, and three Pacific. Subgenus 1. _Thalassicollarium_, Haeckel. _Definition._--Membrane of the central capsule structureless, only perforated by innumerable very small radial pores. {19}1. _Thalassicolla pellucida_, n. sp. Spherical body very soft, transparent, clear and colourless, without any pigment. Central capsule soft, hyaline, with a thin, structureless, not areolated membrane. Diameter of the central capsule about three times that of the nucleus, one-fourth to one-sixth that of the jelly-envelope. Nucleus delicate, transparent, with one single central nucleolus, about one-third its diameter. Protoplasm of the central capsule contains only small, pellucid, densely packed globules (vacuoles?), no oil-globules. Extracapsular body quite transparent, without pigment or oil-globules, only composed of the large alveoles imbedded in the jelly-cover, and of the fine protoplasmic network between them. No xanthellæ. _Dimensions._--Diameter of the central capsule 0.8 to 1.2, of the nucleus 0.3 to 0.4, of the calymma 3 to 6 mm. _Habitat._--Cosmopolitan, Canary Islands, Haeckel; Cape, Australia, Pacific, Challenger; surface. 2. _Thalassicolla spumida_, n. sp.