Page:Scientific results HMS Challenger vol 18 part 1.djvu/899

thin spiral lamellæ and give to the whole shell more solidity. Often these beams form imperfect radial septa, by which the spiral cavity of the turnings is divided into a variable number of chambers. But these chambers never become so regular and perfect as in the analogous nautiloid Polythalamia.

In many Lithelida the growth of the shell reaches a certain limit, concluding with the formation of a superficial latticed lamella of lentelliptical or nearly spherical form. In many other forms of the family this seems not to be the case; but these may possibly be younger forms, afterwards reaching the same limit.

The network of the shell in the Lithelida is commonly quite irregular, and so variable that its special conformation has usually no value in the determination of the species. The surface of the shell is often covered with radial spines, which are sometimes arborescent.

The central capsule seems always to preserve the same lentelliptical form (or triaxial ellipsoid) as in all other Larcoidea. With the increase of growth it encloses successively a larger part of the spiral cortical shell, but on the outside is constantly protected by the last turnings of the spiral, or by the lattice-lamella of the surface.

Synopsis of the Genera of Lithelida.

II. Subfamily Spiremida. Central medullary shell simple, spherical or lentelliptical.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	Surface of the cortical shell smooth or thorny, withou radial spines,	304. Spirema.
		Surface of the cortical shell covered with numerous simple or branched radial spines,	305. Lithelius.
II. Subfamily Larcospirida. Central medullary shell double, trizonal or Larnacilla-shaped.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	The transverse girdle turns around the principal axis,	306. Larcospira.
		The lateral girdle turns around the sagittal axis,	307. Pylospira.
		The sagittal girdle turns around the transverse axis,	308. Tholospira.
		Both wings of the transverse girdle turn around the principal axis in an opposite diagonal direction,	309. Spironium.

Subfamily 1. Spiremida, Haeckel, 1881, Prodromus, p. 464.

Definition.—Lithelida with simple, spherical or subspherical, medullary shell.

Genus 304. Spirema,^[1] Haeckel, 1881, Prodromus, p. 464.

Definition.—Lithelida with simple, spherical or subspherical, medullary shell, and lentelliptical or subspherical, spirally constructed cortical shell; surface smooth or thorny, without radial spines.

1. Spirema = Convolution, turning; σπείρημα.