MYTILIMERIA. 3C3 



America ; examples also occur among the great Islands of 

 the Indian and Philippine Archipelagoes. 



Species of Lyonsia. 



auatina, Gray. hyalina, Couth. 



arenosa, Moll. nitida, Gould. 



bracteata, Couth. Norvegica, Chem. 



Brasiliensis, Couth. solemyalis, Lam. 



brevifrons, Sow. striata, Bosc. 

 elongata, Gray. 



Sub-gen. entodesma, Philippi. 



Shell thin, Saxicava-shaped, slightly inequivalve and gaping, 

 covered with a thick epidermis. Hinge edentulous; a semi- 

 circular process in each valve containing the cartilage. 



Chilensis, Phil. 



Genus MYTILIMERIA, Conrad. 



Shell sub-oval, inflated, fragile, equivalve, surface of 

 valves covered with a deciduous, thin, yellowish epidermis, 

 beaks sub-spiral. Hinge edentulous, with a slight linear 

 cavity under the beaks ; cartilage with a narrow ossicle in 

 front. Muscular impressions two, rather small; pallia! 

 line with a broad, obtuse sinus. 



Syn. Byssonia, Valencie?i?res. 



Ex. M. Nuttallii, Conrad, pi. 96, fig. 4, 4, a. 



These shells differ from the perforating Modiohe in the 

 form and position of the muscular impressions, and in the 

 sinuated pallial line. The typical species is a thin, fragile 

 shell of a white colour, and inhabits the Coast of Califor- 



