Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/394

 380 FORAMINIFERA later-formed chambers do not merely surround the earlier, but are piled up more or less regularly upon them ; and we find such aggre gations spreading over the surfaces of shells, or clustering round the stems of zoophytes. Two of the most remarkable modifications of the planorbuline type, which strikingly illustrate the extremely wide range of variation among Foraminifera, are Polytrcma and Orbitolina. The former, which was described by Lamarck as a coral, under the name of M-illcpora rubra, is a very common para site upon shells and corals from the wanner seas, sometimes spread ing itself out horizontally in the form of a depressed cone, some times rising up from a small base in an arborescent growth, and sometimes developing itself into knobby excrescences. In all these varieties, the structure of the individual chambers, and their mode of communication with each other, remain essentially the same, the diversity in the aggregate forms being simply due to the want of any definite plan of increase. The Orbitoliwv (Lamarck), which occur abundantly in certain beds of Chalk, seem to have been gigantic forms of the planorbuline type, represented at the present time by some comparatively minute organisms found 011 the Aus tralian coast, which commence life as Planorbulinoc, and then, by the sequential addition of chambers both above and below, as well as at the margin, come to acquire a sugar-loaf or a globular form. The size of the globular examples ot fossil Orbitolincc, which, like the flints of chalk, are often found in Tertiary strata, varies from that of a small pea to that of a large bullet. There is a spread ing Orbitolina in the Chalk, which sometimes attains the extra ordinary diameter of four inches. These forms are of peculiar in terest, as showing the tendency of the Foraminiferal type to that indefinite Zoophytic extension, of which we have the highest example in the .bozoon to be presently described. The highest forms of the Jtotaline type (which are confined to tropical seas) correspond with those of the &quot;Nummuline&quot; in the complexity of the organization of their shells, which only re veals itself to microscopic investigation. Each chamber has its own complete chamber-wall, so that the partitions which separate two contiguous chambers are double; and between these we find an additional deposit of calcareous substance, which the writer has designated as thb &quot; intermediate skele ton.&quot; This deposit is channelled - through by a set of canals distributed upon a regular plan, and al together forming a canal-system &quot; ( fig. 18). These do not communicate direct ly with the chambers, but give passage to sarcodic extensions formed by the coal escence of pseudo- podia that have pass ed through the pores of showing flic inter- the proper chain- F G - 18. Section of Rotalirt fteccarii, showing .r vill Ami cnnal s y stun1 &amp;lt;*&amp;gt; * f - in the substance of the i .[-.iiib. AUU mediate skeleton ; &amp;lt;/, tubulated chamber-wall. the development of this &quot;canal-system is exactly proportionate to that of the &quot;in termediate skeleton,&quot; it may be affirmed with some confidence that, as the formation of the latter proceeds, in the first instance, from the sarcodic expansion which covers the exterior of the chamber-walls, so it is continued by tho prolongation of that Flo. 19. Calcarina laid open: a, tabulated wall of chalnbers; !&amp;gt;, intermediate skeleton, extending into outgrowths; c, channelled by canal-system. expansion into the &quot; canal-system.&quot; Now, in Calcarina, (so named from its resemblance to a spur-rowel, fig. 19) we find an extraordinary development of this intermediate skeleton, which so completely envelops the original rotalian shell, that nothing can be seen of its chambers externally, save towards the termination of the last whorl of the spire, and which extends itself into club- shaped outgrowths that have no connexion with the chambered spire except through the canal system. There is no other recent Foraminifer at present known in which the &quot; intermediate skele ton&quot; and the &quot;canal-system&quot; attain such a remarkable develop ment ; and it was under the guidance of the structure of Calcarina, as previously elucidated by the writer s microscopic inquiries, thnt Dr Dawson was first led to recognize the &quot; canal-system &quot; in Eozoi-&amp;gt;i mnadensc, and thus to determine its Foraminiferal character as will be presently explained. Another very interesting type which must be placed in the &quot; globigerine &quot; series on account of the coarseness of the perfora tions of its shell-wall, but whose spiral mode of growth diffeis entirely from that of ordinary Eotalians, is the Fitsidina of the Carboniferous Limestone (fig. 20). This bears so strong a resem blance hi form to the Ahcolina of the &quot;Milioline&quot; scries (fig. ](&amp;gt;), that it was for a long time associated with that type, although differing from it in the opening of its chambered interior by a .single fissure along the middle of the lip, instead of by a row of pores. The interior, instead of being minutely divided into a number of separate chamberlets, is found to consist essentially of a symmetri cal spire of simple chambers, each of which extends on either side FIG. 20. Section of Fuiulina Limestone. into an &quot;alar prolongation,&quot; analogous to that of other spirally growing Foraminifera. But these alar prolongations, instead of overlapping the preceding whorls, as in Ismnmulites, are prolonged in the direction of the axis of the spire, those of each whorl pro jecting beyond those of the preceding, so that the shell is elongated with every addition to its diameter. These alar extensions are con stricted at intervals, so as to form liifcar series of partially divided chamberlets, and frequently take a somewhat irregular course ; so that when the shell is traversed by sections (fig. 20) these reveal an aggregation of chamberlets (like those of the investing whorls of Nammulites, fig. 24), in which no definite arrangement can ut first be traced. The Family Nummulinida, of which the well-known fossil Xitm- mulite is a characteristic example, includes the greater part of the largest and most elaborately constructed of the &quot;vitreous&quot; Foram inifera. Various plans of growth prevail in this family ; but its Fir.. 21. IntevnaVcast-of Polystomella craticulata : a, retral processes, proceeding . from the posterior margin of one of the segments; b, 6, smooth anterior margin of the same segment; t, c 1, stolons connecting successive segments mid uniting themselves with the diverging branches of the meridional canals ; rf, d&amp;gt;, &amp;lt;P, three turns of one of the spiral canals ; e, e 1, e 2 , three of the meridional canals; /,/ 1 ,/ 2, their diverging branches. distinguishing characters consist in the completeness of the wall tltat surrounds each segment of the body (the partitions between the chambers being double), in the density and fine tabulation of the shell-substance, and in the presence of an &quot; intermediate skeleton&quot; with a &quot;canal-system&quot; for its nutrition. This last, however, is altogether wanting in the smallest and more feebly developed ex-