as to produce very characteristic sculpturing of the surface. [Illustration: FIG. 3.--Various forms of Calcareous Foraminifera. 1, _Cornuspira_. 11, _Cristellaria_. 2, _Spiroloculina_. 12, _Globigerina_. 3, _Triloculina_. 13, _Polymorphina_. 4, _Biloculina_. 14, _Textularia_. 5, _Peneroplis_. 15, _Discorbina_. 6, _Orbiculina_ (cyclical). 16, _Polystomella_. 7, _Orbiculina_ (young). 17, _Planorbulina_. 8, _Orbiculina_ (spiral). 18, _Rotalia_. 9, _Lagena_. 19, _Nonionina_. 10, _Nodosaria_.] Compound or "polythalamic" shells derive their general form largely from the relations of successive chambers in size, shape and direction. This is well shown in the porcellanous _Miliolidae_. If we call the straight line uniting the two ends of a chamber the "polar axis," we find that successive chambers have their pylomes at alternate poles; but they lie on different meridians. In _Spiroloculina_ (fig. 3, 2) the divergence between the meridians is 180 deg., and the chambers are strongly incurved, so that the whole shell forms a flat spiral, of nearly circular outline. In the majority, however, the chambers are crescentic in section, their transverse prolongations being termed "alary" outgrowths, so that successive chambers overlap; when under this condition the angle of successive meridians is still 180 deg. we have the form _Biloculina_ (fig. 3, 4), or with the alary extensions completely enveloping, _Uniloculina_; when the angle is 120 deg. we have _Triloculina_, or 144 deg., _Quinqueloculina_. Again in _Peneroplis_ (figs. 3, 5, and 4) the shell begins as a flattened shell which tends to straighten out with further growth and additional chambers. In some forms (_Spirolina_, fig. 22, 3) the chambers have a nearly circular transverse section, and the adult shell is thus crozier-shaped. In others (which may have the same sculpture, and are scarcely distinguishable as species) the chambers are short and wide, drawn out at right angles to the axis, but in the plane of the spiral, and the growing shell becomes fan-shaped or "flabelliform" (figs. 3, 5, 4, 2). This widening may go on till the outer chambers form the greater part of a circle, as in _Orbiculina_ (fig. 3, 6-8) where, moreover, each large chamber is subdivided by incomplete vertical bulkheads into a tier of chamberlets; each chamberlet has a distinct pylomic pore opening to the outside or to those of the next outer zone. In _Orbitolites_ (figs. 5, 6) we have a centre on a somewhat Milioline type; and after a few chambers in spiral succession, complete circles of chambers are formed. In the larger forms the new zones are of greater height, and horizontal bulkheads divide the chamberlets into vertical tiers, each with its own pylomic pore. [Illustration: FIG. 4.--Modifications of _Peneroplis_. 1, _Dendritina_; 2, _Eu-Peneroplis_.]