8 GASTEKOPODA. 



It Is equally easy to determine the natural position 

 of the bivalves without the presence of the animal ; 

 for the ligament is always placed on the dorsal sur- 

 face of the animal, and the mouth is placed on that 

 side of the apex of the valve, or umbo, which is be- 

 fore the ligament. Consequently, if a bivalve shell 

 is placed on the table, with its hinge-side upjiermost, 

 and with the ligament towards the observer, the shell 

 will be in its natural situation, and the sides of the 

 shell will agree with the sides of the observer. 



It is to be remarked that Linnaeus, and the natu- 

 ralists of his school, described what is here called the 

 front of the shell as the back, the left valve as the 

 right, and vice versa ; and Lamarck in general (but 

 not universally) followed the same rule. The me- 

 thod above described is, however, so obviously cor- 

 rect, and every other so liable to confusion from the 

 want of a sound foundation, that it cannot fail, 

 sooner or later, to be universally adopted. 



Class I. GASTEROPODA. 



The adult animal furnished with a more or less dis- 

 tinct foot, placed under the body, for crawling on 

 the ground or the surface of the water. Having a 

 distinct head furnished with eyes and tentacles, they 

 are usually protected by one or by two unequal-sized 

 valves, the largest being conical, spiral, and enclosing 

 the greater part of the body, and the smaller forming 



