LYONSIA. 213 



LYONSIA, TuRTON. 



Shell inequivalve, more or less inequilateral, both valves 

 convex. Surface often striated and invested with an epi- 

 dermis. Hinge without teeth, but provided with a mov- 

 able testaceous ossicle, connected with each valve by car- 

 tilage ; ligament internal. Muscular and palleal impressions 

 weakly marked, the latter sinuated. 



Animal oblong ; mantle closed, except a fimbriated 

 opening anteriorly for the passage of a tongue-shaped 

 grooved foot ; siphons very short, united nearly to their 

 orifices, which are fringed. Labial tentacles triangular, 

 small. 



Although there are few better marked genera among 

 the European bivalves than this, the nomenclature and 

 synonomy of our native species are singularly varied and 

 confused, owing, chiefly, to the comparative rarity of the 

 shell, and the imperfect state in which it not unfrequently 

 found its way into cabinets. The genus, however, is an 

 excellent one, and important in a systematic point of view, 

 since it constitutes an excellent transition from Pandora to 

 Anatina and its allies. The peculiar nacreous structure of 

 the shell, first examined microscopically by Dr. Carpenter, 

 links it rather with the former tribe, an union which is 

 borne out by features in the anatomy of the animal, espe- 

 cially the peculiarities of its respiratory system. 



The species of Lyonsia are few, but distributed through 

 arctic, temperate, and tropical seas. Some forms from 

 the Indian seas indicate a generic affinity with Sphenia. 

 The history of its fossil members has not yet been clearly 

 made out. Some tolerably well marked species occur in 

 strata of the Cretaceous epoch. 



