45 



brides ; Lamarck observesj that in its exterior aspect 

 it much resembles Blya arenaria. 



Lamarck has quoted the Mya dedivis, Pen., as a 

 synonyme of his Jlnatina myalis, which Blainville 

 considers the type of Schumacher's genus Periploma ; 

 it is very different from that genus as figured and de- 

 scribed by Schumacher. 



Deshayes, in his Tertiary tables, gives two recent 

 and four fossil species of Thracia. 



LUTRARIA. 



Shell equivalve, inequilateral, oblong or subovate^ 

 gaping at the extremities ; hinge with two diverging 

 teeth, united above, and a large deltoid cavity 

 situated obliquely beneath them ; lateral teeth none ; 

 ligament internal ; muscular impressions two ; palleal 

 impression with a profound sinus. 



OBSERVATIONS. 



This genus was separated by Lamarck from the 

 Linnean Mactrse and included in his family Mac- 

 TRACEES. Cuvier has grouped it with Mya in his 

 family Inclusa. Lamarck describes eleven recent 

 species, and Deshayes enumerates six in the Ter- 

 tiary beds : two species occur in the Eocene of South 

 Carolina and Alabama, the L. papyria and petrosa, 

 nobis. 



