FAMILY 5. MACTRACEA. 59 



Example. 



PI. XL. Fig. 1 to 4. 



Cleidoth.erus chamoides, Sowerby, Genera of Shells, No. 32. 

 Chama albida? Lamarck. 

 Cleidotharus albidus, Deshayes. 

 Chamostrea (chamoides?), De Roissy. Gray. 



Family 5. MACTRACEA. 



Testa plerumque inaequilateralis, lateribus vel clausis vel hiantibus. 

 Ligamentum partim vel omnino internum. 



The Mactracea form a very distinct and well-defined family, though, 

 from the peculiar modification of character in many of them, consider- 

 able confusion has arisen in the subdivision. The careful exposition of 

 this family by Deshayes, in his notes to the new edition of Lamarck, has 

 however added considerably to our knowledge ; and we think that the 

 adoption of his genus Mesodesma will materially assist the arrange- 

 ment. 



The Mactracea have generally an inequilateral shell, either closed or 

 gaping, and the ligament is always partially or altogether internal. We 

 include eight genera in this family, as follows : 



Lutraria. Mesodesma. 



Mactra. Ungulina. 



Gnathodon. Amphidesma. 



Crassatella. Cumingia. 



i2 



