xxii THE HALL OF SHELLS. 
times completely burying themselves in sepulchers of their 
own making. See tke work of pholades in the columns of 
the ancient temple of Serapis, at Pozzuolis. 
CHAPTER X VII.—Among the most beautiful of shells 
_are those of Olivide. The markings of some suggest 
delicate cuneiform and picture writing. The external sur- 
face of these is pictured entirely different from the deco- 
rations on layers just below. These mollusks have the 
ability to dissolve away earlier formed volutions. Study 
olives and cone shells in Structural and Systematic Con- 
chology, by George W. Tryon. 
CHAPTER X VIII.—Shells were once parts of the mantle 
or delicate films secreted by and thrown off from it: these 
harden, and unite with other tissues previously thrown off. 
Observe these layers joined to each other as shown in the 
more solid shell—the Cassides, for example. 
The color of shells also the secreting work of the man- 
tle ; tints dependent upon light. Still, how is a mystery. 
Observe under valves of shells where light has not 
reached. 
CHAPTER XIX.—See Land Shells, published by Ameri- 
can Tract Society, for lessons upon the eyes of snails, and 
apparent affection among them. Sounds are produced 
by some varieties of sea slugs. Auditorial nerves are dis- 
covered in some shellfishes. Their whole surface ex- 
tremely sensitive. 
CHAPTER X X.—In the family Cypreide we find the 
beautiful porcelain shells which change their exquisite 
markings with different stages of growth. In this family 
members of the genus Ovulum are beautifully enameled, 
but lack the coloring of the Cyprea. 
