Allonautilus perforatus is a species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of nautilus

Nautilus

Nautilus is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in two genera, the type of which is the genus Nautilus...

 native to the waters around Bali

Bali

Bali is an Indonesian island located in the westernmost end of the Lesser Sunda Islands, lying between Java to the west and Lombok to the east...

, Indonesia

Indonesia

Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...

. 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

Umbilicus (mollusk)

The umbilicus of a shell is the axially aligned, hollow cone-shaped space within the whorls of a coiled mollusc shell. The term umbilicus is often used in descriptions of gastropod shells, i.e...

. 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

Periostracum

The periostracum is a thin organic coating or "skin" which is the outermost layer of the shell of many shelled animals, including mollusks and brachiopods. Among mollusks it is primarily seen in snails and clams, i.e. in bivalves and gastropods, but it is also found in cephalopods such as the...

) characteristic of A. scrobiculatus. Maximum known shell diameter is around 180 mm.