Mystery snails lay the same type of egg case as Apple snails, but they are
two different snail types. ~ jan

Pond Newbie said
Are these some kind of egg? If so, of what?

Apple snails, you can check out these plant eaters here.
http://www.applesnail.net/content/ecology.php
HTH -_- how

~ jan (Do you know where your water quality is?)