INTRODUCTION. Vll 



strength of shell fish, found, by means of a machine 

 made for the purpose, that a weight of twenty-five 

 pounds would force the valves of an Area barbata, 

 while it required fifty-nine pounds to open a Spon- 

 dylus. 



Among the animals inhabiting shells, some are 

 oviparous, and others viviparous. Of the oviparous, 

 some are isolated, and covered with a crust, like the 

 eggs of birds, such as the terrestrial snails ; others 

 are included in a gelly, which unites them together, 

 such as the aquatic snails ; while others, again, 

 as the whelks, deposit their eggs in membranous 

 bags, of an oval, or spherical shape, generally 

 grouped in bunches, like grapes, of a pale straw 

 colour, with each a small hole in the side. 



The young of Bivalves are contained between the 

 tracheae of the parent, that is to say, the leaves of 

 their beard. They are covered with shell, even be- 

 fore they leave the body of the mother. The young 

 of shells which are designed to move from place to 

 place leave their eggs very neatly formed ; but those 

 which are to remain fixed during their lives, are at 

 first covered with a mucilaginous matter, which 

 sticks to whatever it touches, and thus forms the 

 first adhesion, which is afterwards strengthened by 

 the stony juice, secreted from the body of the animal. 



