The extreme top of the shell is called the /apex/, and it may be either sharply pointed or obtuse. Each turn is called a /whorl/; the last one, and always the largest, is called the /body-whorl/, the others collectively forming the /spire/ of the shell. When the animal first emerges from the egg it is always provided with one or two whorls, which generally may be distinguished at the apex as smoother than the rest of the spire, and a faint line may be discovered where the shell began its growth after birth. These [pg344] first whorls of embryonic growth are usually referred to as the /nepionic whorls/. The interstices between the successive whorls are called /sutures/, and these lines of juncture may be shallow or deep according to the convexity of the whorls. The sculpturing of the whorls is often of great complexity, and exhibits /nodules/, /varices/, /ribs/, /revolving striæ/, /spinous processes/, and /lines of growth/. It frequently happens that there is a complete absence of any sculpturing whatever, the whole surface of the shell being perfectly smooth, save for the faint lines of growth to be detected only under a magnifying-glass. The outer edge of the mouth or aperture of the shell is called the /outer lip/. This may be more or less thickened, expanded, or simple. The inner lip is called the /columella/, and may be greatly extended anteriorly. When the mouth of the shell is round, and there is no notch or indentation either above or below, it is said to be /continuous/. The /anterior canal/ is for the accommodation of the siphon of the animal, which, as noted above, may be very long or almost entirely absent; therefore the character of the shell generally indicates whether or not the animal itself possesses a long siphon. There is in some forms a notch at the upper extremity of the mouth, which is for the accommodation of the anal discharges. When present it is called the /posterior canal/.