Pleiosaurids in which tooth ornament is reduced: the dentary bears 24 to 26 teeth on each ramus; the premaxillae bear six teeth each, of which the first (most anterior) is small and the second to sixth are large; the parietals for a sagittal crest; the paraoccipital process of the exoccipital-ophisthotic is of moderate length; the occiptal condyle is not ringed by a groove, and extends onto the pedicles of the exocciptials; there are about 55 pre-sacral vertebrae, of which usually 32 are cervical; the cervical vertebrae have relatively amphicoelous centra, the length of which rarely exceeds the height; the clavicles are triangular and well developed, lile visceral to the ventral rami of the scapulae, and meet in the mid-line; the interclavicle is absent or rudementary; the coracoids meet the scapulae in the ventral mid-line in 'adults'; the width across the posterior cornua of the coracoids exceeds the interglenoid width in 'adults' by up to 40%; the elements of the pectoral girdle tend to fuse in old individuals; there are normally only two epidodials in the manus; a foramen only appears between the epipodials showing advanced ossification.

Greek, kryptos, 'hidden', Greek, kleid- (klets), 'key, clavicle' + us: Hidden clavicle.