"Though there was a flounder for sale on a fishmonger's barrow"

A flounder is a marine flatfish found in both northern Atlantic and Pacific waters. Depending on the species, mature flounders may weigh anything between 20 pounds (9 kilos) and 600 pounds (272 kilos).

Plaice, sole and halibut are all types of flounder.

 

Flowery flounder (Bothus mancus)
GNU Free Documentation LicenseFlowery flounder (Bothus mancus) - Credit: Mila Zinkova, Wikimedia Commons