292 COMPARATIVE ZOOLOGY OF CHORDATES 



the higher forms, however, it often happens that the post- 

 orbital and squamosal bones no longer touch one another. 

 The result of this is that the inferior temporal fossa now 

 extends up between them and is bordered above by the parietal 

 bone. From the mere fact that it touches the parietal it must 

 not be mistaken for a superior temporal fossa. This enlarged 

 type of inferior temporal fossa is present in the higher Thero- 

 morph reptiles, and in the mammals. A fossa of this type is also 

 found in the Sauropterygia, or Plesiosaurs. Here again, 

 although the fossa is bordered by the parietal, it is probably 

 an inferior temporal fossa which has extended in the manner 

 just described. For this reason, the Sauropterygia are usually 

 classed as Synaptosauria, close to the Synapsida. Synapsida 

 usually have two condyles. 



The Parapsida have a single superior temporal fossa on 

 each side, lying above the post-orbital and squamosal bones, 

 and the supra-temporal bone appears to have been retained. 

 To this group belong the Ichthyosaurs and the Squamata, 

 which latter consist of the Lacertilia and Ophidia. In the 

 Lacertilia, the bar beneath the lateral temporal fossa has been 

 much reduced by emargination from below.* The result of 

 this is that there is very little roofing left over the temporal 

 region, and the quadrate, which still retains the otic process 

 abutting against the paroccipital process of the auditory capsule 

 (see p. 104), becomes uncovered and loose. The quadrate is 

 therefore capable of movement relatively to the squamosal 

 and to the brain-case. This condition, which is called strepto- 

 stylic, is associated with the fact that the upper jaw can move 

 relatively to the brain-case, which arrangement enables the 

 animal to open its mouth with a gape wider than would 

 otherwise be possible. 



An extreme case of the streptostylic condition is found in 

 the Ophidia or snakes. Here the postorbital bar and the 

 temporal bar are completely broken down, so that the temporal 

 region is uncovered. The quadrate has lost its connexion 

 with the auditory capsule, and is only indirectly articulated 



* Some authorities prefer to regard the Lacertilia as derived from 

 Diapsida which have lost the inferior temporal bar. 



