It's possible that:
* There's a misspelling: The term might be a mistranslation or a misspelling of a real shell type. Can you provide any further context, such as the origin of the term or its intended meaning?
* It's a local name: "Ugpan" could be a local name for a specific type of shell in a particular region or language.
To help you find the English name, please provide more information about the context of the term "ugpan shells."