Diagnosis: Species of Selenopemphix featuring crests on the longitudinal cingular sutures. On these septa as well as on some other sutures, numerous short, slender, conical spines arise, with pointed tips.
Dimensions: Holotype: 57 x 49 μm (l x d); Paratype: 54 x 46 μm (l x d). Average dimensions (n = 9) 59.4 μm (st. dev. 5.3 μm) x 55.5 (st. dev. 6.9 μm).
Description: Protoperidinioid cyst much wider than high. The autophragm is smooth, featuring short (2–5 µm), sharp, conical spines that are aligned on crests which follow sutures. The apical horn and the two antapical horns are blunt. The archeopyle involves the second intercalary plate (2a), which is somewhat rounded and often remains adnate. The tabulation is typical protoperidinioid whenever discernable. The sutures between the precingular and cingular plates as well as those between the cingulum and the postcingular plates bear short (2–4 µm) crests along which the spines are aligned. These septa seem interrupted at least, but not exclusively at the cingular plate boundaries. The sulcus is indent, giving the cyst outline in polar view the shape of a bent ellipse.
Stratigraphic distribution: In the type section, Selenopemphix brinkhuisii occurs between mid-C13n to mid-C12n. This equates to between 33.4 Ma and 30.8 Ma.
Geographic distribution: To date, Selenopemphix brinkhuisii sp. nov. has only been recognized in the type section.
Affinities: Selenopemphix brinkhuisii sp. nov. differs from other spinose Selenopemphix species such as S. brevispinosa, S. coronata, S. crenata, S. dioneacysta, S. islandensis, S. selenoides S. warriensis and S. undulata in having spines also on crests representing postcingular and precingular sutures. Furthermore, crests of S. brinkhuisii are discontinuous, and not continuous such as at S. undulata and S. selenoides.