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Ancestor: Viviparus viviparus (Common River Snail)

Size: Species vary in size between just 2-3 centimetres in shell diameter at maturity, to up to 15 centimetres shell diameter size for the largest species. Due to continued growth throughout life, particularly long-lived outliers can sometimes exceed typical mature sizes by a large gap.

Dietary Needs: Subsists on biofilms containing various microbes including bacteria, and algal scum and fallen detritus that it scrapes off rocks and surfaces with it's radula, particularly that which gathers on it's shell.

Life Cycle: Their ancestor Viviparus viviparus was unusual amongst freshwater snails for having distinctly sexed individuals and for the female birthing live young that hatched internally. These snails have inherited the same methods of reproduction. Like their ancestor, males have a shorter right tentacle with a rounded tip, where the penis is contained within, whereas the female has two identical long, pointed tentacles.

Females are fertilized by males and give birth to anywhere between 30 - 300 offspring depending on the species, in the spring either in their second or third year, how early depends on latitude and when the season permits fast growth, and on species within this group. Females don't live long past giving birth, but they hang on a bit longer to complete this process than the males, who typically only live for a little more than a year, because once they have fertilized females their job in life is done.

Other: Compared with their ancestors, their shells are much smoother and flatter, with fewer large or deep grooves or bumps for their main predator, shell-punching mice, to grip. The mice use their incisors similar to a paper-punch to pierce the shell, while using their forepaws as hands to hold the shell steady in place. The paws of the mouse have vacuum-adhesive pads that help them avoid slipping on wet rocks but also help them grip smooth shells. This is partly why the snail evolved to coat it's shell in a thick but slippery mucous, a response to combat the literal grip mice had over them. This innovation made it very difficult for the mice to apply any pressure with their teeth without sending the shell flying, and even holding the snail could become challenging to the mouse. This helped curb the death rate of snails discovered by mice.