Page:Scientific results HMS Challenger vol 18 part 1.djvu/771

Rh a true phylogenetic relation. But it is remarkable that we do not find further forms of development in this family, by multiplication either of the arm-chambers (further growth in the three perradii) or of the concentric chambered rings (in the periphery of the disk margin).

The central capsule of the Pylodiscida is constantly flat, discoidal, and enclosed between the two sieve-plates of the surface. Its form is either circular or triangular.

Definition.— with a simple, spherical or lenticular, central chamber, surrounded by three simple arm-chambers, which are separated by three notches or gates.

Definition.— with a simple, spherical or lenticular, central chamber, surrounded by three simple arm-chambers. Notches between the three arms open.

The genus Triolena is the most simple form of all Pylodiscida, and must be regarded as their common ancestral form, from an ontogenetic as well as a phylogenetic point of view. The small shell is composed of a simple, spherical or lenticular, latticed, central chamber, and of three simple, surrounding equal arms, which are also simple latticed chambers, lie in the equatorial plane, and are separated by three equal angles or open gates.