The case is spiral because the shark's oviducal gland moulds it that way. Each egg is about 15 centimetres long with two flanges spiralling around the cone. The embryo develops inside on its yolk for roughly ten to twelve months, one of the longer incubations among sharks, before a fully formed pup emerges.
The wedging behaviour is inferred from where the eggs are found, jammed into crevices at angles that would be hard to achieve by drifting, and from the case's soft state at laying. Eggs left loose on the bottom are frequently eaten, and empty cases wash ashore in numbers each spring.
The adults have their own oddity. Port Jackson sharks have sharp cutting teeth at the front and broad flat crushing plates at the back, a dentition that gives the genus its name Heterodontus, or different teeth, and lets one animal both grip and mill hard-shelled prey.