POLTCISTINA. 155 



increase of the successive chambers, and by their 

 ventricose or parallel-sided form. A very prevalent 

 type in the Pacific is that of the OrbituUtes, which 

 very much resembles a coin in its circularity, flatness, 

 and comparative thickness ; and a species from the 

 Australian coast equals a sixpence in size. This 

 pretty shell is made up of a number of thin concentric 

 circles, each of which is composed of many flattened 

 chambers, communicating by minute orifices with 

 those of its own range, and also of the ranges within 

 and without it. In this type the central or primal 

 cell is comparatively large, of pear-like form, and is 

 almost surrounded by a secondary chamber, which is 

 far larger than any of the rest. 



Very closely allied to the Foraminifera are the Poly- 

 cystina ; shell-bearing animals, of even more extreme 

 minuteness, which have been only recently made 

 known, but which are found to exist, in considerable 

 abundance, in the oceanic deposits, and to be still 

 more numerous in certain geological formations. They 

 have been recognised by Ehrenberg in the chalks and 

 marls of the Mediterranean coasts, as Sicily, Greece, 

 and North Africa ; and in the diatomaceous deposits 

 of Bermuda and Virginia : in the island of Barba- 

 does, the rock of a very extensive district has been 

 found by the great Prussian microscopist to be almost 

 entirely composed of Polycystine shells, with a slight 

 admixture of Foraminifera and Diatomacece, and 

 with calcareous earth, which seems to have been de- 

 rived from the decomposition of corals ; all oceanic 



