Looking at the picture on the left hand side of the 2nd row, it appears that the two sepal appendances connect below the apex of the sepal. The apex of the sepal is the point furthest away from where the sepal attaches to the flower. There is a significant-sized ridge at the apex of the sepal and the appendances are attached beneath that ridge.
In addition, the seeds of Common Evening Primrose are 1.2 to 1.8 mm long and the seeds of Northern Evening Primrose are 1.8 to 2.2 mm long. As you can see from the picture on the right of the second row, it appears that the seeds are approximately 1.8 to 1.9 mm long when measuring from the center of each black hash mark that represents 1 mm.
Finally, a variation of Northern Evening Primrose is Oakes Evening Primrose (Oenothera oakesiana). Oakes Evening Primrose, which is found primarily from Massachusetts and south to Virginia, is covered with minute, stiff and appressed gray or white hairs. But this plant has an intermingling of short and long hairs which is commonly seen in the Northern parviflora variation.
