Yesterday evening I found Garlic Snail finally (Oxychilus alliarius)
One of the ID features of this species is that it exudes a strong Garlic smell upon handling (or fingering as quoted in many places). Once Dr Boden had stopped sniggering, he found the compound responsible: n-propanethiol (below). Nothing like the compound allicin that gives garlic its smell (also below).
n-propanethiol is very similar in structure to n-butanethiol (just an extra CH2 in the chain) and n-butanethiol is one of the compounds that give Skunks their smell. Rearrange the chain of n-butanethiol slightly and you get the compound below - tertbutylthiol. This is the compound that is added to domestic gas supplies so you can smell the gas (methane is odourless).
For anyone who has had the pleasure of eating white truffles (or even truffle oil), you will know that they also smell a bit like domestic gas but this is due to another (mixture) of compounds, one of the principle compounds being 2,4-dithiapentane (below). All of these compounds are very volatile and contain sulfur (they are known as VOSCs - volatile organic sulfur compounds) and the slight differences in their structures give them different odours. In this family is also DMS, dimethylsulfide, the attractant they mix with chum to attract seabirds.
For a really good example of how little changes in chemical structure can give completely different smells, look at the two compounds below:
Enough geekiness - I'll try and stick to organisms for a bit.