A(.T.I J HAI.A. 27 



any length of time. Besides, it being the object of the 

 conchologist to collect, arrange, and preserve some of the 

 greatest beauties which nature has presented to us, in the 

 form of shells, this object could be effected only by an 

 arrangement founded on the shells themselves. 



58. Without going farther into the general subject of this 

 order, we shall examine a few individuals as types of their 

 general organization, so far as this is known. 



ACEPHALA. 



59. This term means without head, and common ex- 

 amples exist iii the muscle, oyster, and scollop. These 

 are "bivalve shells, the two valves being united at the back 

 by a hinge, and connected by teeth which lock into each 

 other. Beside these, the two shells are connected by 



Fisr. 20. 



means of a strong ligament in form of a short pillar, a 6, 

 represented in Fig. 20, which are the two valves of a spe- 

 cies of unio, or fresh-water clam. These ligaments are 

 very distinct in the common species of venus, called round 

 clam,, seen in all the fish -markets of the Atlantic states. 

 These ligaments hold the two valves together with great 

 force, so that when the animal is alive it is difficult to 

 separate them without a knife. 



60. There is, also, in all bivalve shells, a cartilage, gen- 

 erally of a dark color, situated between the two valves at 

 the hinge, the office of which is to force them asunder. 



