Shell spiral, in most cases with an elevated, coneshaped spire which consists of a few whorls, frequently with spiral sculpture; aperture narrowed above; columellar margin as a rule with 1 or 2 folds; the foot bears a horny operculum.
Animal retractable into the shell, with fairly narrow creeping foot; head shield large, posteriorly produced into 2 triangular lobes, below it without externally visible sense organs. Radula with more or less numerous rows of plates, the form and size of which are highly variable; stomach without plates; intestine short. The mantle has a posterior glandular, tube-shaped process. The gill consists of a single triangular lamella, the tip of which is forwardly directed, anterior to its attachment is a small roundish osphradium; the heart, positioned on the left, has the auricle lying in front of the ventricle. Anterior to the esophagus, nervous system with a ring in which the cerebral and pleural ganglia are fused; visceral commissure distinctly twisted; the supraintestinal ganglion lying on the left, the subintestinal ganglion on the right. Penis not invaginable, connected with the hermaphroditic gland by an internal sperm duct, so that 2 genital openings are present.
This family is intermediate between prosobranchs and opisthobranchs; as in the former, an operculum is present, the ventricle lies behind the auricle, the visceral commissure is distinctly twisted, and the penis not invaginable; on the other hand, the family approaches opisthobranchs mainly in the buccal armature, the form of the gill and osphradium, the head shield, and the sense organs. The anatomy of most of the genera is still unknown.