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252 As might be suspected in such forms, the number of presacral vertebrae is reduced.

The temporal vacuity is bounded as in the plesiosaurs, and also in some theriodonts. The maxillae are large, the nares situated rather far back, perhaps an adaptation for grubbing in the mud after invertebrates. Possibly there was a moderate adaptation in Placodus for life in shallow water.

The placodonts were reptiles of considerable size, perhaps eight or ten feet in length, undoubtedly slow in movement, and with a heavy skull, as have all shell-eating reptiles.

Until more is known of the skeleton, the group may remain in an independent position, though there is little in the structure of the skull that would entitle them to an ordinal rank; shell-eating animals with crushing teeth occur in various orders.

. Upper Triassic. Placodus Agassiz (Anomosaurus Huene), Placochelys Jaekel, Cyamodus Meyer, Europe.