A topographic feature of the periflagellar area of some prorocentroids (desmokonts). It is a thecal extension of a periflagellar plate that can lie adjacent to the periflagellar pores (flagellar and auxiliary) and/or surround one or both pores (e.g. Prorocentrum maculosum). The periflagellar collar can be flared and/or protuberant (e.g. P. formosum), or appear as a winged spine (e.g. P. caribbaeum). Oftentimes, this feature can only be viewed via SEM.