aperture or peristome of the shell may be simple or variously modified. An outer and an inner (columellar) lip are generally recognized. These may be continuous with each other, or may be divided by an anterior notch. This, in some types (Fusinus, etc.) it is drawn out into an anterior siphonal canal, of greater or lesser length. An upper or posterior notch is present in certain taxa, and this may result in the formation of a ridge or shelf next to the suture (Clavilithes). An outer (lateral) emargination or notch, sometimes prolonged into a slit occurs in certain types (Pleurotomidae, Pleurotomaridae, Bellerophontidae, etc.), and the progressive closing of this slit may give rise to a definitely marked slit band. In some cases the slit is abandoned and left as a hole (Fissurellidae), or by periodic renewal as a succession of holes (Haliotis). The outer emargination is often only indicated by the reflected course of the lines of growth on the shell. On the inside of the outer lip, various ridges or plications called lirae are sometimes found, and these occasionally may be strong and tooth-like (Nerinea). Similar ridges or columellar plicae or folds are more often found on the inner lip, next to the columella or central spiral twist. These may be oblique or normal to the axis of coiling (horizontal), few or numerous, readily seen, or far within the shell so as to be invisible except in broken shells. When the axis of coiling is hollow (perforate spire) the opening at the base constitutes the umbilicus. The umbilicus varies greatly in size, and may be wholly or in part covered by an expansion or callus of the inner lip (Natica). Many Recent shells, when the animal is alive or the shell is freshly empty, have an uppermost shell layer of horny, smooth, or hairy epidermis or periostracum, a proteinaceous layer which sometimes is thick enough to hide the color markings of the surface of the shell. The periostracum, as well as the coloration, is only rarely preserved in fossil shells. The apertural end of the gastropod shell is the anterior end, nearest to the head of the animal; the apex of the spire is often the posterior end or at least is the dorsal side. Most authors figure the shells with the apex of the spire uppermost. In life, when the soft parts of these snail are retracted, in