406 MANUAL OF THE MOLLUSCA. 
guishing 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 a pallial sinus. 
4, Form of the foot. 
5. Structure of the branchie. 
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 mequality 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 yalue.* 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 Jridina 
(6), and especially in Dreissena (3) it is more closed than in the most nearly allied 
genera. 
2. Mulleria (6) and Tiidacna (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 
Tsoarca (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. 
10. Hinnites (1), Attheria (6), Myochama and Chamostrea (19) are irregular. 
11. Pecten is free, byssiferous, or fixed: Arca 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 Lithodomus and Ungulina—boring shells—have the foot like 
Mytilus and Lucina. 
12. Novaculina is a river Solen, and Scaphula afresh-water Arca. ~- 
