CONCHIFERA. 



253 



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



7. Position of the lujament, 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 

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

 too implicit reliance on sirigle characters. Groups, to he natural, must be 

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

 animal 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) ; pallial impression simple. 



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

 line simple or obsolete. 



FAMILY I. OsTREiP^. 



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

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

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

 edentulous. 



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



* 1. Cardita and Crassatclla (Fam. 13) have the mantle more open.'whilst in 

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

 allied genera. 



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



3. Zerfa(4)and^dacraa(10)have a pallial sinus; Anapa (1(5) has none. 



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

 is adapHvehj modified. 



3. Diplodonta (11) has four gills. 



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



7. Crassalella (13) and Semele (16) have an internal ligament; in Solenella ^nd 

 Isoarca (4) it is external. 



5. Anodon (16), Adacna, Serripes (10), and Cryptodon (II) are edentulous. 



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

 arcinella (7) is equivalve. 



10. Hlnnites (1), uEtheria (C), Myochamaund Chamoslrea (19) are irregular. 



11. Peclen 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 jEtheria, thoagh fixed, has a large 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. 



