Page:The Osteology of the Reptiles.pdf/74

56

The extreme aquatic adaptations of the plesiosaurs have caused certain modifications in the structure and relations of the bones of the skull that are unique among reptiles.

The general shape of the skull seems to bear a definite relation to the length of the neck, always shorter in the long-necked forms, slender, sometimes very slender in the short-necked types. The premaxillae are always greatly elongated, extending back at least as far as the front part of the orbit, in the long-faced kinds even to articulate with the parietals, above or between the frontals. The alveolar border also is [short?] shut. The maxillae are much more elongate than in the icthyosaurs and phytosaurs. There are no teeth in the palate. The nasals have never been certainly determined; possibly they are fused with the frontals, which extend as far forward as the external nares, forming the inner border. The prefrontals are small; the lacrimals have been identified in a few forms only; they appear to be absent in some. The postfrontals are probably present in all. The orbits are bounded below by the jugals and maxillae. The quadratojugals are conceded to be absent in all plesiosaurs. The single large temporal opening is bounded below by the postorbitals and squamosal, above by the parietals, which are more or less elevated in the middle in a crest. There is a pineal foramen. The squamosals, in some at least, join each other back of the parietals on the upper surface of the skull. There are no interparietals, tabulars, or supratemporals.

On the occipital surface the supraoccipital is excavated more or less by the foramen magnum, which may extend to the roof, dividing the bone. The paroccipitals are always fused with the exoccipitals. The brain-case is more or less open in front on the sides, the postoptics either reduced or absent. The stapes is large and stout.