3^ COSCUYLIX—DITHYRA. 9. 



9. LYO^SlX—^IJfiTON. 



Testa transversa, infequivalvis, iwequilateralis, latere antko 

 hiante. Cardo dente transverso cordato mobili commuui, 

 valvas foveolis transtersis coadunanie : lateralibus nullis. 

 Ligamentum internum. 



Shell transverse, inequivalve, inequilateral, open at the anterior 

 end. Hinge with a transverse heart-shaped moveable tooth, 

 common to both the valves, and uniting them by ^ transverse 

 cavity in each : lateral teeth none. Ligament internal. 



This genus is remarkable for the peculiar structure of the 

 hiuo-e, 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 substance of the shell itself, as iu all other 

 known shells furnished with teeth; but is an independent process, 

 moveable with the ligament, and may be entirely detached from 

 either or both the valves: consequently when the valves are 

 opened, it is found sometimes in the right valve and sometimes in 

 the left, as the ligament may casually loosen, exhibiting the form 

 of a somewhat elevated transverse tooth. 



It is of an oblong heart-shaped figure, appearing like a piece 

 of enamel fixed to the point of junction between the valves, with 

 the pointed end directly under the beaks. 



We have dedicated it to our worthy correspondent, Mr. Lyons, 

 of Tenby, who first presented it to the notice of the British 

 naturalist. 



