UNio. 315 



Occasionally the hinge-margin is furnished with an irregular 

 series of tubercles, but these, also, are often obsolete. As 

 the name implies, they are found abundantly on river-banks, 

 after being washed up by the tide. 



The genus Unio includes the greater number of those 

 which are distinguished by thick and massive teeth. No 

 particulars of interest have transpired concerning them, 

 and yet the Unio occidens is instanced by Dr. Carpenter, 

 in his valuable Treatise on Shells, as beautifully illustrat- 

 ing the arrangement of the outer and inner layers. The 

 vertical section brings into view the two substances of which 

 the shell is composed, and most curious are they : the one 

 an outer or prismatic layer, the other a nacreous lining, 

 made up of numerous laminae. Lines are also obvious, in- 

 dicative of successive formations, and proving that at every 

 enlargement of the shell the whole interior is lined with a 

 fresh pearly lamina, in immediate contact with that which 

 preceded it. The number of such laminae, therefore, like 

 successive circles in the trunks of trees, indicate the number 

 of enlargements which the shelly structure has undergone. 



A beautiful variety of patterns, minute and exquisitely 

 varied as those produced by the kaleidoscope, are discover- 

 able in different shells. The prismatic cellular structure 



