INTRODUCTORY CHAPTER. 



21 



teeth, c; and lateral*, or side teeth, D d. e e are the 

 muscular impressions, and F the pallial impression, which 

 marks the size of the animal, being formed by the border 

 of the pallium or mantle. When there is a flexuous line 

 or sinus in the pallial impression, it denotes that the 

 animal had siphons which were contractile ; sometimes a 

 third scar is visible ; this is the spot where the muscles of 

 the foot were attached. G G are the sides of the shell, 

 which is called an equilateral shell when they are equal in 

 length, and inequilateral when they are unequal. H is the 

 ligament, which is horny, and by its elasticity tends to 

 keep open the valves. This is counteracted at the animal's 

 pleasure, by the muscles attached within the shell. The 

 ligament is sometimes internal, and sometimes external, 

 or both at the same time. 



Mollusca form two great divisions, Univalves and Bi- 

 valves : the first comprising all those, the shells of which 

 are formed of one single piece, as the snail; and the latter, 

 those which have shells consisting of two valves, or dis- 

 tinct pieces, as the oyster. Each of these is again divided 

 into classes and orders, each order into families, each family 

 into genera, and each genus into species. These will be 

 described in succession, according to the systematic ar- 

 rangement of Dr. Philippi f , extending as far as the different 

 genera : the limits of this work, which is intended but as 

 an introduction to the science, will not allow of a descrip- 

 tion of each species, and a mere dry list of their names 

 would be uninteresting. Keference can easily be made by 

 the reader, who has mastered the subject as far as it will 

 be pursued here, to the many large and scientific works 



* From latiiS) Lat., a side. 



t Handbuch der Concliyliologie und Malacozoologie. Halle, 1853. 



c 3 



