It is any plane parallel to the sagittal plane.
Parasagittal is correct.
Sagittal or parasagittal
parasagittal
The sagittal plane is a vertical plane through the longitudinal axis dividing the body into left and right portions. If the animal is bisected straight down the midline, the section is called a "midsagittal" section. If the animal is sectioned away from and lateral to the midline, the section is referred to as "parasagittal".
It is any plane parallel to the sagittal plane.
This is called a sagittal plane if divided this way with symmetrical left and right halves. If the section produces asymmetrical halves, this is a parasagittal section.
Though flexion and extension can occur in a variety of planes depending on the body's position, if you have a body in anatomical position, then all flexion and extension occurs on the sagittal plane.
There are a variety of features but one is the number of fenestrae in the skull. Mammals also have more differentiated teeth than reptiles. Furthermore, you can look at the gait. Mammals have a parasagittal stance, while most reptiles have a sprawled stance.