Understanding OS process ids

Hello,

Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

Running a 3 node RAC

When running the query :

SQL> select sid,serial#,process from gv$session where sid=328;

  SID   SERIAL# PROCESS

------ ---------- ------------------------

  328     12309 20120:29048

  328     27107 5916:8684

I expect to see the OS process ID in the 'process' column but I receive two numbers separated by a colon. Neither number relates to an ongoing process on the OS

What does this colon separated number represent please ?