Concerning the concept of extinct classes of Mollusca: or what may/may notbe a class of mollusks

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.