an eighth means an eighth of an ounce (i'm pretty sure, i never really learned the metric system, but i know it's three and a half grams), which is a measure of volume. so, as grapefruity said, it depends on the volume of each individual mushroom.
ounces when used ireference to most drugs is not liquid ounce; it is a unit of measurement referencing dry weight. IE a pound as in a pound of beef is 16 dry ounces. a dry ounce is roughly 28 grams....thus an 1/8th is 3.5gs. IT DOES NOT DEPEND ON VOLUME. as to the OP it depends on type of mushies and size.
sorry, sorry, i was under the impression that i knew everything there for a second. at least i got the 3.5 right.