INTRODUCTION. 17 



or the shells; hut at last it was determined that it should 

 be on the latter ; because few of the marine shells that were 

 then found had the animal in them, prior to the time when 

 dredging was resorted to. The greater part of shells in 

 many collections at the present day, have been found upon 

 the beach, divested of their animal: they having been cast 

 ashore by the agitation of the sea in storms, and the inhabi- 

 tant decayed before they were picked up. 



Many shells are very different in their young and adult 

 states, both in form and colour; the student ought, there- 

 fore, to make himself well acquainted with the generic 

 characters, to enable him to discriminate the difference. 

 But, indeed, this is often a difficult matter, even to those 

 who have studied the science for many years. 



In almost all the species of the genus Cypraea, there is con- 

 siderable difficulty in distinguishing them from the Volutae 

 and Bullae, in their young state; the denticulated lip being 

 the last part of the shell which the animal forms. The 

 Strombus genus wants the expanded lip in the young shell : 

 from which circumstance it is frequently taken for a species 

 of the genera Buccinum or Murex. 



The Linnaean arrangement of shells consists of three 

 orders, viz. Multivalve, Bivalve, and Univalve. 



The first order, Multivalve, is made up of shells consist- 

 ing of more shelly parts or pieces than two. Every part of 

 a shell which is connected with a corresponding part by a 

 cartilage, ligament, hinge, or tooth, is called a valve of such 

 shell. 



The second order, Bivalve, is made up of shells having 

 two parts or valves, generally connected by a cartilage: for 

 example, the Cockle and Muscle. 



The third order, Univalve, is made up of shells complete 

 in one piece: for example, the Periwinkle and the Whelk. 

 This order is subdivided: first, into shells with a regular 

 spire, and those without a spire. 



The first order consists of three genera: Chiton, Lepas, 

 and Pholas. 



The second order contains fourteen genera: Mya, Solen, 



