406 MANUAL OP THE MOLLttSCA. 



guisliing tliese groups and tlie genera of bivalves are tlie fol- 

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



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



2. Number and position of muscular impressions* 



3. Presence or absence of a pallial sinus. 



4. Form of the foot. 



5. Structure of the branchice. 



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. 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 oi* 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.) 



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 ex- 

 ternally ; pallial line simple or obsolete. 



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

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

 genera. 



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



3. Leda (4) and Adactia (lO)Jiave a pallial sinus ; Ann-pa (16) has none. 



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

 ajdaptivcly modified. 



o. Diplodonta (11) has four gills. 



6. Pearly structure is variable even in species of tlie same penns. 



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 Cri/ptodon (11) are edentulous. 



9. CbrZ>M/rt (18) and Pandora (19) are more inequivalve than their allies ; Chama 

 arcinella ( 7 ) is equfvalve. 



10. Hinnites (1), ^theria (6), Myochmia and Chamostrea a9) are iiTegular. 



11. Pec^ere is free, byssifcrous, or fixed : Area free or byssiferous. This cliaracter 

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

 form of tlie foot, as Lithodomus and Unyulina — boring shells— have tlie foot like 

 Mytilus and Lticina. 



12. Aovaculina is a river Solen, and Scaphuia a fresh-water Area. 



