THRACIA. 



365 



side, being possibly misled by tbe fact that each lamina, 

 as described by Clark, is divided into two parts by a deep, 

 oblique furrow which gives the appearance of two branchiae. 

 There appears also to be some mistake about the tri-lobed 

 foot, so that the only difference between the two forms is in 

 the irregular valves and vertical cartilage-processes in 

 Rtqncola, which unite face to face when the valves are 

 closed. About seventeen species of Thracia have been 

 described from various parts of the world; China and the 

 Eastern Seas, Greenland, the United States, Norway, the 

 British Islands, the Mediterranean, and the Canaries har- 

 bouring species. 



Species of Thracia. 



Conradi, Couth. 

 convexa, Wood. 

 corbuloides, Desh. 

 Couthouyi, Si imp. 

 curta, Coiir. 

 declivis, Venn. 

 granulosa, Adams and Reeve. 

 magnifica, Jonas. 



plicata, Desh. 

 pubescens, Pultn. 

 rugosa, Conr. 

 similis, Couth. 

 squamosa, Cpr. 

 truncata, Migh. 

 villosiuscula, Macgilliv. 



Sub-gen. rupicola, Fleuriau de Bellevue. 



Orifices of siphons simple. Shell irregular ; cartilage-process 

 vertical, emarginate above; pallial line with a small, obtuse, 

 triangular sinus. 



concentrica, Fleur. de Bellev. distorta, Turt. 



VOL. II. 



3 B 



