CONCHIFERA. 253 



6. Microscopic structure of the shell, (v. p, 38.) 



7. Position of the ligament, internal or external. 



8. Dentition of the hinge. 



9. Equality or inequality of the valves. 



10. Regularity or irregularity of form. 



11. Habit; free, burrowing or fixed. 



12. Medium of respiration, fresh or salt-water. 



A few exceptions may be found, in which one or other of these characters 

 loes not possess its usual value.* Such instances serve to warn us against 

 coo implicit reliance on single characters. Groups, to be natural, must be 

 based on the consideration of all these particulars on " the totality of the 

 inimal organization." (Owen). 



SECTION A. ASIPHONIDA. 



Animal unprovided with respiratory siphons ; mantle-lobes free, or united 

 at only one point which divides the branchial from the exhalent chamber 

 (cloaca) ; pallia! impression simple. 



Shell usually pearly or sub-nacreous inside; cellular externally; pallial 

 line simple or obsolete. 



FAMILY I. OSTREID^. 



Shell inequivalve, slightly inequilatural, free or adherent, resting on one 

 valve ; beaks central, straight ; ligament internal ; epidermis thin ; adductor 

 mpression single, behind the centre; pallial line obscure; hinge usually 

 edentulous. 



Animal marine ; mantle quite open ; very slightly adherent to the edge 



* 1. Cardita and Crassatella (Fam. 13) have the mantle more open,^whilst in 

 Iridina (6), and especially in Dreissena (3) it is more closed than in the most nearly 

 allied genera. 



2. Mulleria (6) and Tridacna (9) are monomyary. 



3. Leda (4) and Adacna (10) have a pallial sinus ; Anapa (16) has none. 



4. The form of the foot is usually characteristic of the families ; but sometimes it 

 is adaptively modified. 



5. Diplodonta (11) has four gills. 



6. Pearly structure is variable even in species of the same genus. 



7. Crassatella (13) and Semele (16) have an internal ligament; in Solenella and 

 Isoarca (4) it is external. , 



8. Anodon (16), Adacna, Serripes (10), and Cryptodon (11) are edentulous. 



9. Corbula (18) and Pandora (19) are more inequivalve than their allies; Chama 

 arcinella (7) is equivalve. 



)0. Hinnites (1), JEtheria ((5), Myochama and Chamostrea (19) are irregular. 



11. Pecten is free, byssiferous, or fixed : Area free or byssiferous. This character 

 varies with age and locality in the same species. It does not always depend on the 

 form of the foot, as JEiher.ia, though fixed, Jias akn?ge. foot, and Lithodomus and Un- 

 gulina boring shells have the foot like Mytilus and Lucina. 



12. Novaculina is a river Solen, and Scaphula a fresh-water Area. 



