Serpulorbis arenaria (1) Thylacodes Seashell Gastropoda Sea Snail Worm Shell Vermetidae

Serpulorbis (Vermeto) arenaria mm 50 x 27 x 18 Thylacodes Seashell Gastropoda Sea Snail Worm Shell Vermetidae, unique piece, as in photos.
Family: Vermetidae.
Common name: Snail Worm or Shell Worm.

Vermetidae (Rafinesque, 1815) are a family of gastropod molluscs of the subclass Caenogastropoda. It is the only extant family of the superfamily Vermetoidea.
The species of this family do not have a shell with a regular spiral structure like that of a typical gastropod shell, but have shells of irregular tubular shape. They are usually cemented to a hard surface. For this reason, the shells resemble the calcareous tubes of worms of the polychaete annelid family Serpulidae. The shell is formed by three layers.
The operculum is formed by a spirally coiled plate of chitin. The genus Thylacodes (also called Serpulorbis) does not have an operculum.
Vermetidae are sessile gastropods. They have developed a special means that allows newly hatched young to cement their shells to the substrate without becoming trapped in the process. The earliest fossil that can qualify as a vermetid is from the Upper Cretaceous age. However, the preservation is not good enough to allow its unconditional acceptance as a gastropod. Some of the Lower Eocene fossils, from the Cuisian of France, are instead indisputably vermetids.