A conchological manual

by G. B. Sowerby

Measurement of Symmetrical Conical Univalves.

The width is measured across the aperture, which is the widest part of the shell. The length (l) from the dorsal part (d) of the aperture to the dorsal part of the whorl (d) on the opposite part of the shell. The ventral part of the whorls is that nearest to the axis, and the dorsal that which forms the outline of the figure.

NON-SYMMETRICAL UNIVALVES.

These are conical, irregular, spiral, or convolute. The conical form is when there is no enrolment of the apex. Although the Patellæ were described as symmetrical, there are several species of Patelliform shells which are not symmetrical. In Umbrella, for instance, the apex is oblique, the shells being placed obliquely on the animal. In the genus Siphonaria, there is a groove on one side, where the brachia or gills of the animal rest. In the genus Ancylus, it will be observed that the apex bends on one side, and the animal is like the Limnæa, which has a spiral shell. The cup and saucer Limpets, or Calyptrædæ, present a group which requires to be described, differently from the symmetrical or true Limpets. Their structure is very curious, and they vary considerably among themselves, some of them being simply conical, others nearly flat, or discoidal, and others more or less spiral. But their principal peculiarity consists in their having a small internal process or plate variously shaped, commonly named their septum.

Septa of Limpets.

The septa of Limpets assume a variety of forms, the principal of which will be seen in the accompanying engravings.

The form from which the group derives its generic appellation is that of the cup-shaped or Cyathiform species (fig. 17). In the Crepidulæ, or Slipper-Limpets, the septum is flat, reaching across the opening, like the deck of a vessel; it is then described as transverse (fig. 20). In Calyptræa Equestris, it has two prominent points, and is described as bi-furcated (fig. 18). In another species, it is a three-sided plate rather spiral at the apex (fig. 19).

Measurement of Cup and Saucer Limpets.