The line marked a, p, ll indicates the direction in which the shell is to be measured for length. a indicates the anterior, p the posterior. The line d (fig. 23), from the apex to the base, is the depth. The line b (fig. 28), is in the direction of the breadth.
Irregular non-symmetrical Univalves.
Serpuliform shells are irregularly twisted (tortuous) hollow tubes, which were formerly considered to have been secreted by a kind of worm, but now known to be the shells of true Molluscs, of a kind not very widely differing from those which have regularly spiral shells. The greater part of these are attached to foreign bodies, or to each other in groups. Some are attached by the whole length of the shell, they are then said to be decumbent. Some of these are coiled round like the Spirorbis, the little white shell seen on the carapace of the Lobster or on leaves of sea-weeds; they are then said to be discoidal; others again, such as the Vermetus, approach more nearly to the spiral form. The deviation from the regular spire only taking place after the few first volutions.
SPIRAL NON-SYMMETRICAL UNIVALVES.
As these constitute the largest class, it will be necessary to dwell upon them in detail. First as to measurement.
The length is measured from the apex, to that part of the aperture a (fig. 24), at the greatest distance from it. The breadth is in the opposite direction. The anterior, or front part of the aperture, is marked a, where the head of the animal protrudes.
Spire of non-symmetrical Univalves.
