148 BIVALVES, 



large tooth is the characteristic of the hinge, but 

 sometimes it is not more than a thickened callo- 

 sity.* Some species are altogether destitute of 

 teeth ; these have a rounded cavity for the re- 

 ception of the cartilage. The gaping of the 

 valves is another distinguishing feature, but it 

 does not always occur. In form also the shells 

 differ considerably ; some are oblong and trun- 

 cated, as if a part of the shell had been cut off ; 

 others are orbicular, and many are angular from 

 the addition of ears at the hinge. The Myae are 

 generally covered with a thick brown or green 

 epidermis ; when this is removed, the surface 

 exhibits a beautiful iridescent mother-of-pearl 

 lustre. Some of the species grow to a great size ; 

 others are remarkable for their thick, solid, 

 and substantial shells, and have in consequence 

 been formed into a distinct genus, called Unio. 

 Many of the Mollusca of this genus burrow in 

 the sand or mud, boring a channel through which 

 they thrust a contracted trunk inclosing two 

 tubes. The My furnish food not only for man, 

 but also for many aquatic birds. 



MYA Truncata.] 

 TRUNCATED GAPER. 



Specific character. Shell ovate, convex, trun- 

 cated at the anterior end, where it gapes consi- 



* Callosity, hardness of skin or flesh, from Latin call MS, which 

 has the same signification ; hence our English word callous. 

 f Plate VII. figure 2. 



