A dinoflagellate sulcus that extends all the way to the posterior end of the cell (also known as antapex). The presence of a sulcal notch makes the dinoflagellate hypocone appear bilobed.
Source:
Wikipedia:Dinoflagellate#Morphology,
ISBN:0632009152,
GOC:at,
http://tolweb.org/Dinoflagellates/2445,
http://www.sms.si.edu/irlspec/Phyl_Dinofl_Glossary.htm
Comment
The term name refers to a taxonomic group to make the label unique with respect to similarly-named anatomical structures. Also, the ventral (front) side of a dinoflagellate cell is the one where the sulcus is located (as opposed to the dorsal (back) side). The term 'sulcal notch' appears mostly in older literature; more recently, indication of a bilobed hypocone is used to express the same.