This is the left atrium of a 60-year-old man who died of unrelated causes. Upon opening the heart, there was a membrane stretched across the left atrium just below the fossa ovale. I had no idea what it was until I looked it up. It’s called “cor triatriatum” and is a membrane that splits one of the atria. If it’s on the left, it’s called cor triatriatum sinistrum, and if it’s on the right it’s called cor triatriatum dextrum. In this person, it was apparently asymptomatic. No other cardiac anomalies were present. It’s a new one on me. I’ve been doing this for 37 years, and I still see things I’ve never seen before.