I performed this ECG earlier this year in an outpatient clinic.

Patient's clinical data:  85-year-old white man; asymptomatic. 

Questions?
(1.)  What rhythm is this tracing showing?
(2.)  What clue/sign is used to differentiate between the two anomalous beats (i.e., 4th and 6th beats).  HINT:  It's named after a part of a certain animal's anatomy.