The sarcode animal itself corresponds exactly with the form and spiral arrangement of the chambers so strongly marked on the exterior of the shell. The segments form a spiral of crescents, smooth on the convex and crenulated on the concave side; and from the latter threads of sarcode proceed, which pass through pores in the inner margins of the partitions, and unite them into one animal. The Polystomella lives in tropical seas; P. crispa in temperate latitudes, and P. striato-punctata inhabits the polar waters; the genus is found everywhere. Although variety of form without specific difference is characteristic of the Foraminifera, it sometimes happens that identity of external form is accompanied by an essential difference in internal structure. Of this the Cycloclypeus is an instance; it is a rare species of nummuline, dredged up from rather deep water off the coast of Borneo. The shell is gigantic, some specimens being two and a half inches in diameter; but its mode of growth is the same with that of the most complicated Orbitolite. It consists of three superposed stages of circular discs, each circle of chambers enclosing all those previously formed. However, each segment of the animal being enclosed in its own shelly envelope, a supplemental skeleton, and a radial, vertical and annular system of canals, prove that the two animals belong to essentially different families of Foraminifera. There are many instances, especially in the Rotaline group, of isomorphism accompanied with generic difference; thus no reliance can be placed on variety of external form, unaccompanied by change of internal structure. An attempt has been made in the preceding pages to describe a few species most characteristic of some of the genera of this multitudinous class; and of those selected a mere sketch of the most prominent features of the animal and its abode is given, that some idea may be formed of the wonderfully complicated structure of beings, which are mostly microscopic specks. Yet the most minute circumstances in the forms of the animals and their shells, with their varieties and affinities, have been determined with an accuracy that does honour to microscopic science. They are now arranged in a natural system by William B. Carpenter, M.D. F.R.S. assisted by William K. Parker, Esq., and T. Rupert Jones, Esq., and published in the Transactions of the Ray Society in 1862. To this admirable work, the author is highly indebted.