. The microscope and its revelations. e seen to proceed the lateral branches (c, c), which, afterburrowing (so to speak) in the walls of the chambers, enter themby large orifices (d). These interseptal canals, and their communi-cation with the inosculating system of passages excavated in the marginal cord, are extremelywell seen in the internal castrepresented in fig. 634. A very interesting modifi-cation of the Nummuline typeis presented in the genusHeterostegina (fig. 635), whichbears a very strong resemblanceto Orb ic nil art in its plan ofgrowth, whilst in every otherrespect it is essentia
. The microscope and its revelations. e seen to proceed the lateral branches (c, c), which, afterburrowing (so to speak) in the walls of the chambers, enter themby large orifices (d). These interseptal canals, and their communi-cation with the inosculating system of passages excavated in the marginal cord, are extremelywell seen in the internal castrepresented in fig. 634. A very interesting modifi-cation of the Nummuline typeis presented in the genusHeterostegina (fig. 635), whichbears a very strong resemblanceto Orb ic nil art in its plan ofgrowth, whilst in every otherrespect it is essentially dif-ferent. If the principal cham-bers of an OjH>rcitrt weredivided into chainberlets bysecondary partitions in a direc-tion transverse to that of t he-principal septa, it would beconverted into a Ifeterostegina, just as a /Itiero/tHs would be converted bv the like subdivision intoan Orbiculina. Moreover, we see in Heterosteyimi. as in (Jrbictttlttu,a great tendency to the opening out of the spire with the advance of. FIG. 635.—Hctiri»,hi/iii,i. NUMMULITES 835 age ; so that the apertural margin extends round a large part of theshell, which thus tends to become discoidal. And it is not a littlecurious that we have in this series another form, Cycloclypeus, whichbears exactly the same relation to Heterostegina that Orbitolites doesto Orbiculina, in being constructed upon the cyclical plan from thecommencement, its chamberlets being arranged in rings around acentral chamber. This remarkable genus, at present only known inthe recent condition by specimens dredged at considerable depthsfrom the coast of Borneo and at one or two points in the WesternPacific, is perhaps the largest of existing Foraniinifera, some speci-mens of its discs in the British Museum having a diameter of twoand a quarter inches. Notwithstanding the difference of its planof growth, it so precisely accords withthe ISTummuline type in every cha-racter which essentially distinguishesthe genus that there
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectmicrosc, bookyear1901
