- Kingdom: Animalia
- Phylum: Annelida
- Class: Polychaeta
- Subclass: Sedentaria
- Order: Sabellida
- Family: Serpulidae
- Scientific name: Placostegus tridentatus
- Norwegian: tretannmark
The tube is made from cemented sand. The ventral end is often erected from the ground. The white, semitransparent mouth ends in three characteristic spikes. The crown is dark red with two brown bands. The body is conical, tapering towards the posterior end. It can be 3 cm long.
It lives on rocky locations or other hard substrate, usually deeper than 25 meter.
It is common in the North Atlantic Ocean and the Mediterranean.