In the notion of classes, all of whose members are extinct, at least two fu
ndamental issues are intertwined: (1) what criteria should be used to assig
n a fossil to the Mollusca; and (2) what level of morphological distinctive
ness is needed to establish a fossil as a representative of an extinct clas
s. One may also ask how Far the morphologic limits of extant classes should
be extended. Several current textbooks recognize at least one class of ext
inct mollusks. There is no consistency in the number of extinct or extant c
lasses in use, nor need there be while the field is still vigorously under
discussion.