406 MANUAL OF THE MOLLUSCA. 



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

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



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



2. Number and j)osition of muscular impressions. 



3. Presence or absence of a pallial sinus. 



4. Form of the foot. 



5. Structure of the hrancluce. 



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, buiTOwing or fixed. 



12. Medium, of respu-ation, 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.) 



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 simjDle or obsolete. 



* 1. Cardita and Crassatella (Fam. 13) have the mantle more open, -wliilst in Iridina 

 (6), and especially in Dreissejia (3) it is more closed than in the most neai'ly allied 

 genera. 



2. Mullcria (6) and 2'ridacna (9) are monomyary. 



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



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

 adoptively 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 

 hoarca (4) it is external. 



8. Aiiodon (16), Adacna, Serripes (10), and CrijpAodon (11) are edentulous, 



9. Cor6M/a (IS) and Pandora (19) are more rnequivalve than their aUies ; C^ma 

 arcinella (7) is equiralve, 



10. Hinnites (I), ^theria (6), IJyochama and Chamostrea (19) are iiregular. 



11. Pecten is free, byssiferous, or fixed: Area free or byssiferous. This cliai-acter 

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

 form of the foot, as Litliodomus and Ungulina — boring shells— have the foot like 

 Mytilus and Lucina. 



12. NovacuUna is a river Solen, and Bcaplada a fresh-water Area. 



