Final answer:
The ridge marking the division between the smooth and rough parts of the atrium is called the Crista terminalis, delineated externally by the sulcus terminalis.
Step-by-step explanation:
The name of the ridge that marks the division between the smooth and rough parts of the atrium is the Crista terminalis. This structure is notable in the right atrium, where it separates the posterior, smooth-walled part of the atrium from the anterior part which has prominent ridges of muscle called the pectinate muscles. Externally, the crista terminalis is marked by a groove known as the sulcus terminalis. The left atrium typically does not have pectinate muscles, except within the auricle.