A topographic feature of some prorocentroids (desmokonts). It is an extension of the intercalary band on the left valve along the anterior margin bordering the periflagellar area (e.g. P. belizeanum and P. hoffmannianum). This feature can be curved, flared, rounded or flattened. Oftentimes, this feature can only be viewed via SEM.
Prorocentrum belizeanum: right valve view. Apical collar on left valve anterior margin borders the periflagellar area.
Prorocentrum hoffmannianum: right valve view. Apical collar on left valve anterior margin borders the periflagellar area.