In general, the crowns of dermal denticles have cusps pointing tailward, which is why a shark feels relatively smooth if stroked from head-to-tail but sandpapery coarse if stroked the other way.
http://www.elasmo-research.org/education/white_shark/scales.htm






























Are you reading the same thing I’m reading? Literally every search result describes shark skin as dermal denticles, or sharp tooth-like scales aligned from front to back, “relatively smooth” in one direction, and sandpaper-like in the other.
Apparently other animals regularly get injured brushing up against them.