76 American Seashells 



stages in the growth of a shell, and their number and position are nsed as 

 identifying characters (see Bursa, the frog shells, pi. 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, pi. 4) or have 

 a parietal shield (see the helmet shells. Cassis, pi. 23V). The inner lip is con- 

 tinuous with the thickened axis or colwnella 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 ca?jal formed at the 

 upper or posterior end of the aperture (see Bursa, pi. 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 (pi. 3). The "stromboid 

 notch" in the conchs is weak but distinct. In the abalones, Haliotis (pi. 2), 

 the slit is replaced by a series of small, round anal holes. Nearly all the tur- 

 rids 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: 

 ( I ) 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. Some- 

 times it is called longitudinal sculpture. Varices, growth lines and the outer 

 lip are axial features. (2) The spii'al 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, pi. 13c), or axially arranged color streaks 

 (as in the Lightning Whelk, Busy con coiitrarium, pi. 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 (pi. 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 znnhilical 

 callus (see Polijiices duplicatus, pi. 5k). About a fourth of our marine spe- 

 cies are umbilicated to some degree or another. 



Teeth (not to be confused with the radular teeth in the animal's pro- 

 boscis or mouth cavity) arc often present in the aperture. The Distorted 

 Shell, Distorsio (pi. 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). 



