xxii THE HALL OF SHELLS. 



times completely burying themselves in sepulchers of their 

 own making. See tl.e work of pholades in the columns of 

 the ancient temple of Serapis, at Pozzuolis. 



Chaptbe XVII. — Among the most beautiful of shells 

 are those of Olividce. 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 XVIII. — 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 XX. — In the family Cyprceidce we find the 

 beautiful porcelain shells which change their exquisite 

 markings with different stages of growth. In this family 

 members of the genus Ovidum are beautifully enameled, 

 but lack the coloring of the Cyprcea. 



