683 MALJ OOLOGY. 



find the shell and the habits of the Mollusks wisely adapted to the 

 situations which they occupy. 



Some that belong to rapid streams, have an exceedingly hard 

 and substantial shell, adapted to contend with the most boisterous 

 elements. Others, by their very levity, are enabled to float on 

 the surface of the water, and offering no resistance, are carried 

 along on the surface of the waves. The Pimvi anchors itself by 

 its byssus to rocks, and thus is secure against all dangers. Oth- 

 ers, as the Nautilus and Argonaut, by adding to the weight of 

 their bark, can descend and seek a shelter in the ocean's bed. 

 There are numerous and beautiful contrivances for their pre- 

 servation. Breaches will, however, sometimes be made in the 

 outer coverings ; but these they have the power to repair, by ex- 

 uding a calcareous matter similar to that with which the shell 

 was first constructed. They are peculiarly abundant in warm 

 climates; being larger and more brilliantly colored, the greater 

 the light and heat to which they are subjected. Including the 

 soft, naked species, as well as those protected by a hard calcare- 

 ous shell, it is believed the number of species will not fall short 

 often or twelve thousand, and this, exclusive of the fossil species, 

 which are thought to be still more numerous. It is said there 

 are scarcely eight hundred living shells found in the Mediterra- 

 nean, or on the French shores of the Atlantic Ocean, but more 

 than twelve hundred fossil shells have been found in that stratum 

 of limestone in which the city of Paris is built, and of which 

 such extensive deposits exist in the neighborhood. " In that 

 single stratum is found, at this day, one third more fossil shells 

 than live on the whole extent of the French shores." (Agassiz.) 



We had designed to follow this general description of the 

 Mollusks, with explanations of the various sub-divisions, after the 

 manner adopted in the preceding sub-kingdoms; but already 

 this volume has swelled far beyond the limits originally assigned 

 to it. Hence, we are constrained to close our account of shells 

 here, referring to the Plates found in this work, and to the Chart, 

 with its numerous figures and explanations, for further illustra- 

 tions; also to Manuals of Conchology, already published, until, 

 if circumstances should hereafter warrant, we may be able to 

 prepare a volume in which the Articulates, Mollusks, and Radi- 

 ates, shall be presented in a manner corresponding in fullness to 

 the view herein presented of the VERTEBRATES. 



What is the Eighth Branch of Zoology ? Of what does it treat ? Under 

 vehat name was the science of shells formerly known? Why was it 



