The marine squamate, Dolichosaurus longicollis, from the Upper Cretaceous (Cenomanian) Chalk deposits of southeast England is redescribed. The elongate neck of D. longicollis is produced by an increased number of cervical vertebrae. Cervical peduncles are elongate, curved and are not fused to the hypapophyses. There is no scapulocoracoid fenestra, the coracoid is not emarginated, and the scapula and coracoid are not fused. The splenial and angular articulate in a ball-and-socket joint similar to that of mosasaurs and Coniasaurus crassidens. The forelimb and pectoral girdle elements show evidence of reduction as compared to the pelvic girdle and rearlimb. Cladistic analysis of six mosasaur taxa, three ‘aigialosaur’ taxa, Coniasaurus crassidens, Coniasaurus gracilodens, and D. longicollis, using 66 characters, found 27 most parsimonious cladograms (MPCs): 122 steps; C.I. 0.648; H.I. 0.352; R.I. 0.669. A Strict Consensus Tree found support for the monophyly of the Mosasauridae and Aigialosauridae; sister-group relationships between coniasaurs, Dolichosaurus, Aigialosauridae and Mosasauridae are an unresolved polytomy. A Majority Rule Consensus Tree finds Dolichosaurus as sistergroup to (C. crassidens, C. gracilodens (Aigialosauridae (Mosasauridae))) in nine (33%) of the MPCs. Lack of support for a more inclusive Dolichosauridae composed of Dolichosaurus (C. crassidens, C. gracilodens) is attributed to the incompleteness of the fossil remains of these three taxa. Presence/absence of a pectoral girdle currently defines the presence/absence of a neck. This definition is insufficient and hypapophyses are found more informative regarding taxic differences and transformational scenarios. The paleobiology of Dolichosaurus is reconstructed as similar to coniasaurs, nothosaurs, and modern sea snakes.