Pisidium (Pea Mussels) but which sub genus?

Pisidium (Pea Mussels) but which sub genus?

Observation - Pisidium (Pea Mussels) but which sub genus?. Description: I have a triving colony of these little cratures in my tiny garden pond. I live on the boundry of the RSPB Pulborough Brooks (No reply from my enquiry there) I wonder if anyone can identify the sub genus and if so, are they rare? My garden pond is rai

I have a triving colony of these little cratures in my tiny garden pond. I live on the boundry of the RSPB Pulborough Brooks (No reply from my enquiry there) I wonder if anyone can identify the sub genus and if so, are they rare?

My garden pond is rainwater fed, deep silty bottom and shaded.