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NOMENCLÁTOR OF GASTROPOD FAMILIES 15
FIG. 4. Number of available family-group names (total 470) based on
genera with a fossil type species, ranked by geological age of the type
species.
In fact, the vast majority of gastropod species
that ever lived on the planet are now fossils.
However, nearly one-fourth (24.6%) of all valid
families occuring in the Recent are slugs, that
do not leave a fossil record, and a still higher
percentage of the modern diversity of Recent
gastropods is not traceable in the fossil record
when one considers the many families with
featureless shells that can only be recognized
anatomically (e.g., the hydrobioid families,
numerous helicoid families, etc.). In the Pa-
leozoic, there is a steady increase in the num-
ber of gastropod families from Cambrian to
Carboniferous, then a crash in the Permian
(Fig. 4). In the Mesozoic, there are more
names with a Jurassic type species than for
any other pre-Tertiary period.
Altogether, the classification recognizes as
valid a total of 611 families, that is 31% of all
1 ,947 potentially valid family-group names, are
currently treated as taxonomically valid. The
other 69% are either synonyms or used as
TABLE 4. Number of Recent and fossil gastropod treated as valid in selected
standard references.
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