Dear Aravinda

Gastropod shells may be the rests of animal died years or decades ago. As long as I understand you must estimate the biomass of gastropods alive at a given time (and at different times to build abundance curves), so you need to know their behaviour, in order to collect them when they are active (by night or rainy weather, for ex.), not to try to collect them when they are hidden.

The gross weight of live animal is probably the first valuable information, completed with the dry matter weight of a representative sample (dried in a ventilated oven  at 60-100 °C until the weight is stable)

the muffle furnace can only give you the ashes, i.e. mineral content (non-organic), not the biomass itself

You probably need also to sort gastropods by species and size...

You have to sort out all specimens at a time, so active searching would provide misleading conclusions. In any ways, you would be better successfull with the larger representative species. 

For forest species, you can use quadrates to take soil samples and sieve them to get an exact profile (of largely smaller species as you might get in your area). You should have a decision of the number of quadrates based on the habitat type and relative size. You might miss arboreal, boulder, or crevice living species this time.

https://www.researchgate.net/.../how_to_measure_the_biomass_of_g...

https://www.ncbi.nlm.nih.gov › NCBI › Literature › Bookshelf