Very characteristic and unmistakable shell. Apex turned back, at spiral, and shell expanding quickly. Whorls separated from the previous one. Surface crossed by close spiral striae. Aperture oval in form: it can be flat but it can also take the form of the substratum to which it is fixed. Surface covered by periostracum, a persistent cuticle, yellow-brown in colour, fringed at the margins of aperture. Shell yellowish in colour pattern. Inside usually white in colour but some specimens can be light pink (variant rosea Daniel, 1883). About two whorls of protoconch are smooth and transparent. Average measures are around 50 mm in diametre and 30 mm in height.

Diagnostic characters
Shell fragile, covered with a flounced periostracum; cap-shaped, with small spiral coil at apex. Aperture a large oval. Animal with proboscis permanently extended from mouth. Operculum absent from foot. Often living on bivalve shells.

Other characters
There is a small, spirally coiled beak of about two whorls, from which the last whorl expands rapidly to form the bulk of the shell. It bears numerous delicate ridges radiating from the apex, and there are also growth marks, some prominent, more or less parallel to the edge of the aperture. The peristome matches the curvature of the surface to which the animal clings. The shell is white, the periostracum straw-coloured. Up to 20 mm long, 20 mm broad.
The proboscis, which is formed from the propodium, is grooved dorsally as in trichotropids, and is mobile. The animals are consecutive hermaphodites: those up to about 4 mm shell length are males and have a curved penis, with open seminal groove, attached behind the right tentacle; larger animals are female and the penis is then reduced to a papilla. The head and foot are yellowish with white speckles.
C. ungaricus is sublittoral, and while it may be found on stones, is mainly attached to shells of living bivalves (Modiolus, Chlamys> Pecten) (Jones, 1949; Sharman, 1956), the gastropod Turritella (Thorson, 1965), and, when young, tubes of Pomatoceros. It uses the proboscis to collect food or the pseudofaeces of its host; it is capable of gathering particles from its own pallial currents, and those living on stones must depend upon this source (Yonge, 1938). The species is widespread off all North Atlantic coasts from the Mediterranean across to eastern America; not, however, in the southern part of the North Sea.
The eggs are held in a mass under the foot and are brooded by the female. From them hatch echinospira larvae. These have a transparent, gelatinous outer shell, almost globular and covered with minute perforations often approximately radially arranged.

It can be found all over the Mediterranean where it lives from a few metres depth up to remarkable depths. It lives fixed to rocks but also epibiont on different bivalves species (such as Chlamys opercularis, Glossus humanus), on brachiopoda (such as Terebratulina retusa), on gasteropoda (such as Turritella sp.) and also on tubicolous of polichetes (Protula). It uses its long proboscis to steal food stored up by its host.