In the past, this gastropod was known as Bellerophon (Pharkidonotus). This is due to subgenera naming. In modern gastropod taxonomy, there are several sub and super names in use, including unranked groups that don’t fall into a classic hierarchy.

The family Bellerophontidae was named by McCoy in 1852 in the publication “Description of the British Paleozoic fossils in the Geological museum of the University of Cambridge”. The order Bellerophontida was named later by Ulrich & Scofield, 1897

Pharkidonotus percarinatus from the Pine Creek limestone in Armstong County, Pennsylvania
CG-0503—Pharkidonotus percarinatus from the Pine Creek limestone in Armstrong County, Pennsylvania. The break in the middle may be evidence of the creature surviving an attack early in life. The shell ornament changes in later life and it seems to have started after the event. Scale bar = 5 mm.
Pharkidonotus percarinatus shell
CG-0503—Pharkidonotus percarinatus from the Pine Creek limestone in Armstrong County, Pennsylvania. Note the central ridge increasing in prominence during growth. Scale bar = 5 mm.

Reference