Page:Scientific results HMS Challenger vol 18 part 1.djvu/812

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604
THE VOYAGE OF THE H.M.S. CHALLENGER.

aggregated without any order, like the Acervulinida among the Foraminifera. In some cases also here the primordial chamber is a trizonal Larnacilla-shell, in other cases it is a simple, subspherical or lentelliptical shell.

The last family, the Phorticida, is formed of irregular Larcoidea, in which a lentelliptical trizonal Larnacilla-shell (as an inner medullary shell) is enveloped by an irregular, latticed, or spongy cortical shell. They can be regarded as abnormalities or irregular deformities of Larnacida or Pylonida.

The central capsule of the Larcoidea is originally lentelliptical and preserves this form, the "triaxial ellipsoid," in the greater number of genera. In some groups it follows the prevalent growth of the shell in the direction of one of the three dimensive axes, and becomes prolonged in this way. In many chambered forms (particularly Tholonida and Zonarida) the growing central capsule gets constricted, corresponding to the constrictions of the shell. In the Soreumida and Phorticida its form often becomes irregular. But in general for the greater number of Larcoidea the lentelliptical form of their central capsule is quite characteristic.

Synopsis of the Families of Larcoidea.


Larcoidea with a regular or symmetrical shell, the growth of which is determined by the three dimensive axes. (Both poles of each axis are equal.) Cortical shell completely latticed, without external gates (or interzonal fissures), without annular constrictions and domes. Medullary shell absent or simple (spherical or lentelliptical), 1. Larcarida.
Medullary shell trizonal or Larnacilla-shaped (composed of three dimensive girdles), 2. Larnacida.
Cortical shell incompletely latticed, with two to four or more symmetrically disposed gates or fissures remaining between latticed dimensive girdles, 3. Pylonida.
Cortical shell completely latticed, without external gates (or interzonal fissures), with two or more annular constrictions, which separate three to six or more dome-shaped protuberances. Constrictions of the cortical shell in diagonal planes; domes in dimensive axes, 4. Tholonida.
Constrictions of the cortical shell in dimensive planes; domes in diagonal axes, 5. Zonarida.
Larcoidea with a symmetrical or irregular shell, either with spiral growth or with quite irregular growth. (Both poles of one axis are different.) Cortical shell with spiral growth. Spiral cortical shell bilateral (with plane spiral), 6. Lithelida.
Spiral cortical shell asymmetrical (with ascending spiral), 7. Streblemida.
Cortical shell with quite irregular growth. Cortical shell simple, with one single chamber, 8. Phorticida.
Cortical shell composed of a number of heaped up or aggregated chambers, 9. Soreumida.