The skull of plesiosaurs showed the "euryapsid" condition, lacking the lower temporal fenestrae, the openings at the lower rear sides. The upper temporal fenestrae formed large openings at the sides of the rear skull roof, the attachment for muscles closing the lower jaws. Generally, the parietal bones were very large, with a midline crest, but the squamosal bones typically formed an arch, excluding the parietals from the occiput. The eye sockets were large, in general pointing obliquely upwards; the pliosaurids had more sideways directed eyes. The eyes were supported by scleral rings, the form of which shows that they were relatively flat, an adaptation to diving. The anteriorly placed internal nostrils, the "choanae", have palatal grooves to channel water, the flow of which would be maintained by hydrodynamic pressure over the posteriorly placed, in front of the eye sockets, external nares during swimming. According to one hypothesis, during its passage through the nasal ducts, the water would have been 'smelled' by olfactory epithelia. However, more to the rear a second pair of openings is present in the palate; a later hypothesis holds that these are the real "choanae" and the front pair in reality represented paired salt glands. The distance between the eye sockets and the nostrils was so limited because the nasal bones were strongly reduced, with many species even absent. The premaxillae directly touched the frontal bones, with the elasmosaurids even reaching backwards to the parietal bones. Often, the lacrimal bones were also lacking.