Thanks for the quick replies. They do appear to resemble H. inspector , although my specimens are certainly more golfball shaped than any of the ones in those photos. Perhaps mine are just fat and old? Here's a few more pics, I'll try to get some better ones in the morning. http://img328.imageshack....

I've had three of these little beasties for a few years, never managed to find out what they are. They were sold to me as 'Snowballs' They are approximately 4" in size.

Can anyone tell me what is the genus & species?