In each of the three above mentioned genera the spiral may be simple or double; it remains simple if only one of both wings of the turning girdle overgrow the other, and this latter remain a simple half girdle (or tube-like wing). Whereas the spiral becomes double if the second wing of the girdle afterwards follow the example of the first wing and now turn around it in the same direction. As this happens in all three genera, we can subdivide them into six subgenera.
A quite peculiar form of spiral growth is produced in Spironium, in which the direction of growth in both lateral wings of the transverse girdle is inverse from the beginning. The left wing grows against the posterior, the right wing against the anterior pole of the principal axis, turning around it in crossed, eight-like spirals. The whole shell afterwards assumes a lentelliptical form.
Commonly between the embracing spiral turnings or convolutions a great number of radial beams is developed, irregularly disposed and often branching; they support the 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. |  | 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. |  | 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. |
| I. Subfamily Spiremida. Central medullary shell simple, spherical or lentelliptical. | ||||
| 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. | ||||
| 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. | ||||