CHAP. X. 



B 1 VA L V E 8 : 

 MACTRA ; Kneading Trough. 



Q. I WILL thank you to tell me how I am to 

 discriminate . the Mactra from other bivalve 

 shells. 



A. By observing that the shells of this genus 

 are inequilateral and equivalve ; that the middle 

 tooth of the hinge is folded with a small hollow 

 adjoining, the lateral teeth being remote, and 

 inserted into each other. 



This genus is easily to be known by observing 

 a hollow in both valves under the beak, be- 

 tween the teeth, serving to hold the cartilage, 

 w-hich is thus wholly external. 



Q. Is there any thing peculiar in the form of 

 the internal cartilage. 



A. Yes. It is generally of a triangular shape, 

 or of one tending to. a three-sided prism, the 

 acute angle of which is placed under the apex 



