,50 



LYONSIA. 



GENERIC CHARACTER. 



Shell inequivalve, inequilateral, open at the poste- 

 rior end ; hinge with a tooth nearly parallel to the 

 hinge margin, common to both valves, to which it is 

 united merely by the ligament, which is wholly in- 

 ternal. 



OBSERVATIONS. 



The curious arrangement of the hinge in Mya 

 J^orwegica of Chemnitz, was first accurately observed 

 by Dr. Turton, who constructed the present genus to 

 receive this interesting shell. The author observes : 

 *^This genus is remarkable for the peculiar struc- 

 ture of the hinge, which consists of a single tooth, 

 placed on the inside equally between the two valves, 

 uniting them by a transverse ligament in each, which 

 is seated in a narrow and rather oblique cavity on the 

 anterior side. This tooth is not a fixed projection 

 from either of the valves, nor formed from the sub- 

 stance of the shell itself, as in all other known shells 

 furnished with teeth, but is an independent process, 

 moveable with the ligament, and may be entirely de- 

 tached from either or both the valves 3 consequently, 

 when the valves are opened, it is found sometimes in 

 the right valve and sometimes in the left, as the liga- 



