Page:American Seashells (1954).djvu/94

76 stages in the growth of a shell, and their number and position are used as identifying characters (see Bursa, the frog shells, pl. 9k).

For the sake of convenience, the part of the lip which is away from the center of the shell or is not next to the axis of the shell is known as the outer lip. Opposite this on the other side of the aperture is the inner lip or parietal wall which may be thickened, armed with teeth (see Nerita, pl. 4) or have a parietal shield (see the helmet shells, Cassis, pl. 23v). The inner lip is continuous with the thickened axis or columella of the shell about which the whorls are developed. In many kinds of marine gastropods, especially the murexes, the columella extends forward and forms the tube-like anterior siphonal canal. In a few genera there is a small posterior canal formed at the upper or posterior end of the aperture (see Bursa, pl. 9k).

The outer lip in a few genera has a very characteristic notch or slit. It is longest in the very rare, large Pleurotomaria shell (pl. 3). The “stromboid notch” in the conchs is weak but distinct. In the abalones, Haliotis (pl. 2), the slit is replaced by a series of small, round anal holes. Nearly all the turrids are recognized by their “turrid notch” on the upper portion of the outer lip. The Keyhole Limpets, Fissurella, have reduced the slit to a single small hole which is located at the apex of their cap-shaped shells, although in their young stages the slit is well-developed at the edge of the shell (see fig. 5).

The sculpturing on the exterior of the shell—ribs, nodules, cords, threads, indented lines, pits, spines, etc.—are grouped into two basic types: (1) The axial sculpture, that is, any markings, ribs or lines which run across the whorl in line with the axis of the shell or from suture to suture. Sometimes it is called longitudinal sculpture. Varices, growth lines and the outer lip are axial features. (2) The spiral sculpture, which is spirally arranged in the direction of the suture or in line with the direction of the growth of the whorls. Thus we often speak of spirally arranged color bands (as in the Tulip Shell, Fasciolaria hunteria, pl. 13c), or axially arranged color streaks (as in the Lightning Whelk, Busycon contrarium, pl. 23-0).

The umbilicus is a hole or chink in the shell next to the base of the columella, which is formed because the whorls are not closely wound against each other at their anterior or basal end. The umbilicus may be quite large and deep as in the sundial shells, Architectonica (pl. 4m). Commonly there is a spiral cord in the umbilicus which may terminate in a button-like callus. Some species are differentiated by the size, position or color of this umbilical callus (see Polinices duplicatus, pl. 5k). About a fourth of our marine species are umbilicated to some degree or another.

Teeth (not to be confused with the radular teeth in the animal’s proboscis or mouth cavity) are often present in the aperture. The Distorted Shell, Distorsio (pl. 25z), is an extreme example, but some shells have teeth on the parietal wall only (Nerita) or on the inside of the outer lip (Cassis).