LYONSIA. 65 



the wcdge-sliaped ossiculum lying against that part. Muscular 

 and pallial marks only indicated l)y a more pearly ajipcarance ; 

 they are far within the shell, and the latter has no well-marked 

 sinus. Length, seven tenths of an inch ; height, four tenths of 

 an inch ; breadth, three tenths of an inch. 



Animal with the foot slender and cylindrical, and with a broad, 

 deep groove ; branchial siphon with eight long and eight short 

 cirri, anal with a valve as long as the siphon. QStit7ipson.^ 



It is found thrown upon the sandy shores of Cape Cod, Chelsea, 

 Lynn, and other similar localities. Its fragile structure is such 

 as to indicate that it could nut live elsewhere than in quiet sand. 

 In April, 1836, the beach at Chelsea was strewed with multitudes of 

 very large and mature ones. Since then I have found only an occa- 

 sional specimen. Whole coast; Grand Manan (^Stimpson); Marble- 

 head Harbor (^Haskell); Eastport, common (^Cooper); St. Anne 

 (^Bcll). 



The ossiculum is almost always detached and lost. Sometimes, 

 when the valves are separated, it adheres to one of them, and then 

 it looks like the tooth of a Mya. When destitute of the ossicu- 

 lum, if reliance were placed upon the hinge alone, the shell w^ould 

 probably be called an Amphidesma, or some undescribed genus. 

 There is no other shell on our coast, however, which presents the 

 radiated wrinkles of the epidermis, together with the pearly lustre 

 of this shell. 



The genus Lyonsia now embraces but three or four species. 

 One of these, the L. Norvegica of Northern European seas, is very 

 similar to ours. But it is distinct. It grows to a much larger 

 size, is more inequipartite, more broadly truncate ; the base is less 

 regularly curved, and is covered by a much stronger and more 

 opaque epidermis. 



Young specimens are very thin, and have a horn-colored exterior, 

 and numerous thin, concentric ridges at the different stages of growth. 



If the valves are miequal, according to the definition of the 

 genus, the difference must be very slight. 



Lyonsia arenosa. 



Shell transversely ovate, ventricose, opaque white, beaks anterior, covered with 

 an ash-colored epidermis to which sand becomes attached. 



Pnndorina arennm, Moller, Index Moll. Grcenl. (1842). 

 Lyonsia (Pandorina) arenosa, MoRCH, Beskr. af Gronl. 90 (1857). 

 5 



