''Polysacos'' is an extinct genus of

multiplacophora Multiplacophora is a stem-group of chitons with a number of plates arranged in 7 rows along the body. They date to at least the Upper Cambrian, but two lower Cambrian fossil A fossil (from Classical Latin , ) is any preserved remains, ...

ns (chitons) known from articulated

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

fossils; its seventeen shell plates are arranged in three rows, with seven iterated units. It demonstrates that multiplacophora are related to modern polyplacophora. It was fringed with a ring of hollow spines resembling those of the Ordovician ''

Echinochiton ''Echinochiton'' is an extinct genus of Ordovician chiton Chitons () are marine molluscs of varying size in the class Polyplacophora ( ), formerly known as Amphineura. About 940 extant and 430 fossil species are recognized. They are also ...

''.

References

Monotypic prehistoric chiton genera {{paleo-mollusc-stub