_Trochospongilla_, _id._, Arch. Naturg. lxi (i), p. 120 (1895). _Tubella_, _id._ (_partim_), _ibid._ p. 128. TYPE, _Spongilla erinaceus_, Ehrenberg. The characteristic feature of this genus is that the rotulæ of the gemmule-spicules, which are homogeneous, have smooth instead of serrated edges. Their stem is always short and they are usually embedded in a granular pneumatic coat. The sponge is small in most of the species as yet known; in some species microscleres without rotulæ are associated with the gemmules. [Illustration: Fig. 23.--A=skeleton-spicule of _Trochospongilla latouchiana_; A'=gemmule-spicule of the same species; B=gemmule of _T. phillottiana_ as seen in optical section from above; B'=skeleton-spicule of same species: A, A', B' × 240; B × 75. All specimens from Calcutta.] I think it best to include in this genus, as the original diagnosis would suggest, all those species in which all the gemmule-spicules are definitely birotulate and have smooth edges to their disks, confining the name _Tubella_ to those in which the upper rotula is reduced to a mere knob. Even in those species in which the two disks are normally equal, individual spicules may be found in which the equality is only approximate, while, on the other hand, it is by no means uncommon for individual spicules in such species as _"Tubella" pennsylvanica_, which is here included in _Trochospongilla_, to have the two disks nearly equal, although normally the upper one is much smaller than the lower. There is very rarely any difficulty, however, in seeing at a glance whether the edge of the disk is smooth or serrated, the only species in which this difficulty would arise being, so far as I am aware, the Australian _Ephydatia capewelli_* (Haswell), the disks of which are undulated and nodulose rather than serrated. GEOGRAPHICAL DISTRIBUTION.--The genus includes so large a proportion of small, inconspicuous species that its distribution is probably known but imperfectly. It would seem to have its headquarters in N. America but also occurs in Europe and Asia. In India three species have been found, one of which (_T. pennsylvanica_) has an extraordinarily wide and apparently discontinuous range, being common in N. America, and having been found in the west of Ireland, the Inner Hebrides, and near the west coast of S. India. The other two Indian species are apparently of not uncommon occurrence in eastern India and Burma. _Key to the Indian Species of_ Trochospongilla. I. Rotules of the gemmule-spicules equal or nearly so. A. Skeleton-spicules smooth, usually pointed _latouchiana_, p. 115. B. Skeleton-spicules spiny, blunt _phillottiana_, p. 117. II. Upper rotule of the gemmule-spicules distinctly smaller than the lower. Skeleton-spicules spiny, pointed _pennsylvanica_, p. 118. 18. Trochospongilla latouchiana*, _Annandale_.