Page:The Osteology of the Reptiles.pdf/45

Rh may not extend in front of the basisphenoid. It has been homologized by Broom with the median vomer of mammals, whence the name prevomers for the paired bones in front, the so-called vomers. Ethmoid (eth) or Sphenethmoid. In the temnospondyl amphibians (Fig. 11 ), between the orbits and in front of the optic foramina, there is a pair of thin bones lying closely below the frontals and united with the parasphenoid below, enclosing an undivided cavity for the olfactory lobes, opening into the nasal and paranasal cavities in front of the orbits. Similar bones have been observed in various cotylosaurs (Fig. 13) and theromorphs, and are probably constant among early reptiles. They have been called sphenethmoids, though they have no immediate relation to the sphenoid. Probably the median ethmoidal plate arose from the fusion of these bones. There was no median ossified interorbital septum in these reptiles, and not probably any median septum. A cartilaginous interorbital septum is present in most modern reptiles but is ossified in none.

The mandible of reptiles was composed primitively of eight, possibly nine, separate bones, differing from that of their temnospondyl antecedents (Figs. 14, 15) only in the loss of one or two slender bones along the inner margin of the teeth, the precoronoid and intercoronoid. All of these, except the postsplenial, known in a single cotylosaur (Fig. 18), have persisted to modern times, though never all in the same reptile, none having more than six, and some but five.

The relations of these bones will be seen in the accompanying