Page:Scientific results HMS Challenger vol 18 part 1.djvu/814

most simple forms of all Larcoidea, we can regard the Cenolarcus as the common ancestral form of this group, having originated from Actilarcus (or the lentelliptical Actissa) by the building of a simple lentelliptical lattice-shell. But it is also possible that a part of the Larcarida (or all?) descend from Larnacida by reduction or loss of the original Larnacilla-shell (compare Cenolarcus triaxonius, p. 607).

Synopsis of the Genera of the Larcarida.

III. Subfamily Cenolarcida. Shell simple, latticed (lentelliptical cortical shell).	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Without radial spines,	266. Cenolarcus.
		With radial spines,	267. Larcarium.
III. Subfamily Coccolarcida. Shell composed of two or more concentric latticed shells (inner medullary and outer cortical).	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Without radial spines,	268. Coccolarcus.
		With radial spines,	269. Larcidium.
III. Subfamily Spongolarcida. Shell spongy, partly or whole composed of a spongy framework.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	With an internal cavity,	270. Spongolarcus.
		Without an internal cavity,	271. Stypolarcus.

Subfamily 1. Cenolarcida, Haeckel.

Definition.—Larcarida with a simple, lentelliptical latticed shell (cortical shell without a medullary shell).

Genus 266. Cenolarcus,^[1] n. gen.

Definition.—Larcarida with a simple, lentelliptical latticed shell, without a medullary shell, without radial spines.

The genus Cenolarcus begins the group of Larcoidea as the most simple form of this suborder. It corresponds to Cenosphæra among the Sphæroidea, to Cenodiscus among the Discoidea, to Cenellipsis among the Prunoidea. The simple latticed shell is distinguished from that of the three other genera by its typical lentelliptical form, a triaxial ellipsoid with three dimensive axes of unequal length. Probably Cenolarcus is the original ancestral form of the Larcoidea, derived from Actilarcus (the lentelliptical Actissa) by the formation of a simple fenestrated shell around the lentelliptical central capsule. But possibly also some species of Cenolarcus may be derived from Coccolarcus or Larnacilla by reduction and loss of the medullary shell (compare Cenolarcus triaxonius, n. sp.).

1. Cenolarcus = Hollow basket; κενός, λάρκος.