ANODOX. 317 



The shell of the Hf/ria possesses many characters in 

 common with that of Unio, but may be, nevertheless, dis- 

 tinguished by its elongated lamellar teeth. 



Eor the next genus, the term Anodon, signifying '^ with- 

 out teeth,^" has been adopted, as expressive of the character 

 by which it is distinguished from the rest of the Naiades. 

 It will be found, however, that the value of this distinctive 

 character is much reduced, as a slight indication of teeth 

 is discoverable in some few shells. The shell is transverse, 

 equivalve, equilateral, generally thin, covered with a green 

 epidermis, and somewhat pearly within. When young, the 

 shell is rather depressed, but increases in convexity as it 

 increases in age. The hinge is linear, or destitute of teeth, 

 with a long external ligament, and the impression of the 

 posterior muscle is always compound. 



IridincB are especially entitled to a generic distinction. 

 The preceding genera have the hinder portion of each 

 lobe of the mantle simple and free, but in this genus it is 

 united, and forms two tubes or syphons. The shells are des- 

 titute of teeth, but the hinge-margin is furnished through- 

 out with thickly-set tubercles. They are covered externally 

 with an olivaceous epidermis, and are internally lined with 

 a reddish pearl, exhibiting more or less of iridescent lustre. 



