Page:The Osteology of the Reptiles.pdf/90

72 joint between the angular and splenial; a similar joint, though less well developed, is found in the monitor lizards. The mandibles are usually united in front by suture but are ligamentously connected in the mosasaurs and some land lizards.

As is seen, there are many variations in the skull of the lizards, more than in many other groups of reptiles called orders.

The skull of snakes differs from that of lizards, especially in the complete closure of the brain cavity in front by descending plates from the parietals and frontals, the former always meeting the basisphenoid below, the latter sometimes interrupted by the coalesced optic foramina; in the constant absence of the postoptics, epipterygoids, and squamosals, the quadrate articulates proximally with the tabular only, which may also be absent. The parietals are always fused; there is no parietal foramen. There is no temporal arch, and, rarely, no ectopterygoid. The premaxillae are small and often edentulous, the maxillae rarely edentulous. The pterygoids and palatines usually bear long teeth. The postorbitals may meet the maxillae below, and there is no jugal.

The vipers (Fig. 59 ) have but one functional tooth attached to the maxilla. It is hollow, with an opening at its base and another near its apex for the passage of venom. Only the dentary is freely articulated in the mandible, the posterior bones closely fused; the two mandibles are usually united in front by ligament only. There is no ossified interorbital septum, and the proötics are largely exposed on the side of the skull.

The mandible of Ophidia has the primitive structure, except that the coronoid appears to be absent or fused, the bone usually so called being clearly the prearticular. The long splenial, as usual in reptiles with a long median symphysis of the mandibles, enters into the symphysis.

The conical teeth of the premaxillae, maxillae, and dentaries primitively were inserted in sockets, but in the more specialized types are rather loosely lodged in grooves.