breadth of the vertebra between the pleurapophyses, or the processes that articulate between the ribs. He considered this to be partially an artifact of preservation, but could not understand how the compression affected only the central portion and not the articular ends of the centrum. Cope recognized this as a natural condition, and considered constrictus to be "a species of Elasmosaurus or an ally". In 1962 Welles considered P.constrictus to be a nomen dubium, given its fragmentary nature. Per Ove Persson retained it as valid in 1963, noting the longitudinal ridge on the sides of the centra as an elasmosaurid trait. In 1995 Nathalie Bardet and Pascal Godefroit also recognized it as an elasmosaurid, albeit indeterminate. Cope discovered another elasmosaurid skeleton in 1876. He named it as a new species, E.serpentinus, in 1877, and differentiated it by the lack of compression in the rear neck vertebrae, the presence of few sessile ribs among the first few dorsals, and the presence of "weak angles" below the front tail vertebrae. Cope had also discovered another large skeleton that bore great resemblance to the known remains of E.orientalis from the black shale of the "Cretaceous bed No.4"; he excavated it with the help of George B. Cledenning and Capt. Nicholas Buesen. In 1943 Welles removed E.serpentinus from Elasmosaurus, and placed it in a new genus, Hydralmosaurus. Subsequently, all Hydralmosaurus specimens were moved to Styxosaurus in 2016, rendering the former a nomen dubium. Williston published a figure of another E. serpentinus specimen in 1914; Elmer Riggs formally described it in 1939. Welles moved this specimen to the new genus and species Alzadasaurus riggsi in 1943. Kenneth Carpenter reassigned it to Thalassomedon haningtoni in 1999; Sachs, Johan Lindgren, and Benjamin Kear noted that the remains represented a juvenile and were significantly distorted, and preferred to retain it as a nomen dubium in 2016. Subsequently, a series of 19 neck and back vertebrae from the Big Bend region of the Missouripart of the Pierre Shale formationwere found by John H. Charles. Cope, upon receiving the bones at the Academy of Natural Sciences, considered them yet another species of Elasmosaurus. The vertebrae were, according to Cope, the shortest among members of the genus (approaching Cimoliasaurus in this condition), but he still considered them as belonging to Elasmosaurus due to their compressed form. He named it E.intermedius in 1894. However, in his 1906 revision of North American plesiosaurs,