Phragmocone

In most nautiloids and ammonoids, the phragmocone is a long, straight, curved, or coiled structure, in which the camerae are linked by a siphuncle which determines buoyancy by means of gas exchange.

Despite this benefit, such a large shell adds to the mass of the animal, and hence is disadvantageous in catching fast-moving prey.

Some nautiloids, such as the Silurian Ascocerida, dropped the phragmocone upon maturity, presumably to increase speed and maneuverability.

The early coleoids and belemnoids adopted a different approach: the phragmocone was retained but became internal and reduced.

[1] There are occasions where trilobites have been preserved within phragmocones, presumably where they crawled in for refuge.