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 Murex 

 temiispina, 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 w&vy. 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 



