Allonautilus perforatus

Allonautilus perforatus is a species of nautilus native to the waters around Bali, Indonesia. It is known only from drifted shells and, as such, is the least studied of the six recognised nautilus species. A. perforatus shows a shell shape and coloration very similar to that of A. scrobiculatus and shares with this species the characteristic open umbilicus. However, it bears highly distinctive shell-ribbing, which is unique among extant ectocochliate cephalopods, and lacks scrobiculate shell sculpture. It is not known whether A. perforatus possesses the thick encrusting layer (periostracum) characteristic of A. scrobiculatus. Maximum known shell diameter is around 180 mm.