decayed, and left the calcareous portion standing in basaltifomi columns, 

 (casts of the cells) not in auy way held together, and separable by the touch. 



The optical irridescence of the nacreous portion of shells, commonly called 

 mother-o-pearl, has been found by the same distinguished physiologist to be 

 produced by the refraction of rays of light falling on the edges of the mo- 

 rocco-like wrinkles of laminary membranous substance. Under a high 

 magnifying power, the nacreous surface exhibits a number of irregularly 

 waved lines winch Sir John Herschell has not unaptly likened to the ap- 

 pearance of the woody layers on the surface of a planed deal board. The 

 brightest nacre is that with which the large Avicula of the Pacific lines 

 its shell ; and the round pearls so highly esteemed by ladies as articles of 

 jewellery, are occasioned by its superabundant flow. Those winch are most 

 highly valued are found within and around the fleshy part of the adductor 

 muscle, arising from disease or irritation. The proprietors of Pearl Fish- 

 eries are said to preserve these animals alive, for the sake of irritating them 

 by the introduction of a stick or other sharp instrument, producing thereby 

 a disorganization of the secreting gland, which causes the pearly nacre to 

 accumulate in little pea-like balls in different parts of the body. 



Beproduction and Habits. 



The reproductive system of the Mollusca is in some viviparous, in others 

 oviparous ; and every kind has its allotted period and uniformity of growth. 

 The shell in embryo, emerging either from the parent directly or from the 

 egg, increases by the gradual deposition of mucus from the mantle in such 

 varieties of form as are suitable to the habits and destiny of the animal. In 

 the Cephalopods, as in the Nautilus, it assumes a discoidal growth, coiling 

 upon itself on a plane with the point of the spire ; in the Gastropods it 

 either revolves obliquely round an axis in the form of an enlarging spiral, 

 as in the Turbo , or it takes a simple conical structure of which the Patella 

 and the Dentalium may be regarded as the extreme forms ; in the Pteropods 

 it presents the shape of a depressed sphere, as in the Hyalaa, or of a cylin- 

 der, as in the Cwviena; whilst in the Bivalves (Lamellihranchiates and 

 BrachiojJOih) the testaceous matter is deposited on a flat, simply convex, 

 or concave surface, as in different species of Pecten, most of which modifica- 

 tions of structure are induced by some corresponding economy in the habits 

 and organization of the subjects. Some shells, such as the Cj/praa, Ma- 

 f/ilus, Pterocera and a few others, present different appearances at different 

 stages of growth. 



The habits of the Mollusca are extremely varied and curious. In the sea 

 they are found dwelling in zones of different depths, either free or attached 

 to foreign substances. About the shore, and at the depth of a few fathoms, 

 where they are most prolific, they are found in mud, among sand and gravel, 



