GASTEROPODS 347 
injured or broken. These places where the lip was thickened, 
as a temporary protection until the young animal decided to 
continue building, appear on the surface of adult shells, and 
persist as varices, “raised lines of growth,” “longitudinal ribs,” 
etc. Such forms of sculptural markings generally occur at equi- 
distant points and preserve a scheme of regularity which would 
indicate that the periods of rest were of seasonal occurrence. All 
irregularities and all surface features of the external shell simply 
reflect certain peculiarities of the mantle margin of the animal; 
all spinous processes, for instance, as in the marvelous Murer 
tenuispina, indicate the existence of finger-like processes extend- 
ing from the mantle margin, which once occupied the hollow 
thorns projecting from the shell. When the outer surface of a 
shell is devoid of any sculpturing and is perfectly smooth, one 
may assume that the mantle margin of the animal was simple. If 
the edge of the lip has a crenulated appearance and the surface of 
the whorls is decorated with revolving ribs, the mantle margin 
was probably folded or wavy. There can be no doubt but that the 
entire mantle surface is provided to some extent with glands for 
the secretion of shelly matter, for if any portion of the shell is 
accidentally injured the animal soon repairs the break with a cal- 
careous deposit. But such repairs are never homogeneous in 
texture with the other parts of the shell of normal growth; the 
patches are never covered externally with an epidermis, and they 
are always devoid of color. The function of the epidermis is to 
protect the calcareous shell from the corroding agents contained 
in sea-water. Hence it is that, when the epidermis is removed, 
“dead shells” exposed to the influence of the water or the weather 
soon lose their brilliancy and luster, and become undesirable for 
specimens either for the cabinet or for study. It often happens 
that living shells, inhabiting a region where, for one cause or 
another, the water is highly charged with impurities of an acid 
nature, are discovered to be badly corroded about the apex, where 
the epidermis is thin and likely to be rubbed off. For this reason 
specimens collected in harbors near large cities or near the mouths 
of rivers are apt to be poor and defective. 
The inner surface of all shells, if not actually nacreous, is 
