Page:EB1911 - Volume 10.djvu/651

 highest vitreous shells, however, each chamber has its complete “proper wall”; while a “supplementary skeleton,” a deposit of shelly matter, binds the chambers together into a compact whole. In all cases the protoplasm from the pylome may deposit additional matter on the outside of the shell, so as to produce very characteristic sculpturing of the surface.

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°, 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° we have the form Biloculina (fig. 3, 4), or with the alary extensions completely enveloping, Uniloculina; when the angle is 120° we have Triloculina, or 144°, 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.

The Cheilostomellidae (fig. 3, 13) reproduce among perforate vitreous genera what we have already seen in the Miliolida: Orbitoides (fig. 10, 2) and Cycloclypeus, among the Nummulite group, with a very finely perforate wall, recall the porcellanous Orbiculina and Orbitolites.

In flat spiral forms (figs. 22, 1, 7; 3, 2, 16, 19, &c.) all the chambers may be freely exposed; or the successive chambers be wider transversely than their predecessors