406 MANUAL OF THE MOLLUSCA. 



guisliiiig these groups and the genera of bivalves are the fol- 

 lowing, stated nearly in the order of their value :— 



1. Extent to which the mantle-lobes are united. 



2. Number and position of muscular impressions. 



3. Presence or absence of sl pallial sinus. 



4. Form of the foot. 



5. Structure of the branchiae. 



6. Microscopic structure of the shell, {v. p. 31.) 



7. Position of the ligament, internal or external* 



8. Dentition of the hinge. 



9. Equality or inequality of the valves. 



10. Eegularity 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 in- 

 stances serve to warn us against too implicit reliance on single 

 characters. Groups, to be natural, must be based on the con- 

 sideration of all these particulars — on ' ' the totality of the 

 animal organisation." (Owen.) 



SECTIOlSr 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 ex- 

 ternally ; pallial line simple or obsolete. 



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

 {%), and especially in Dreissena (3) it is mol-e closed than in the most nearly allied 

 genera. 



2. Midler m (6) and Tridacna (9) are monomj^ary. 



3. Leda (4) and Adacna (lO)Jiave a pallial sinus ; Annpa (16) has none. 



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

 OidajMvely modified. 



o. 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 

 Jsoarca (4) it is external. 



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



9. Cor^M^a (18) and Pandora (19) are more inequivalve than Uieu- allies ; Chama, 

 arcme//(2 (7) is equiValve. ' 



10. Hinnites (1), .Mtheria (6), Myochama and Chamostrea Q9) are irregular. I 



11. Pecfew is free, byssiferous, or fixed: Area free or byssiferous. This cliaractar 

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

 form of the foot, as Lithodomus and Unyulina — boring shells- have the loot like 

 Mytilus and Lucina. 



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



