SKULL. 319 



replaced by downwardly projecting processes of the parietals 

 and frontals, or by a bone known as the epipterygoid (columella 

 cranii). The parietals and frontals are paired or unpaired and 

 there is a squamosal which is usually attached to the parietals. 

 The quadrates which are always present and give articulation to 

 the lower jaw are moveably {Streptostylica) or immoveably 

 (Monimostylica) attached to the skull. They are usually carried 

 by outwardly projecting processes {parotic processes) of the 

 auditory region into which the prootic, opisthotic and exocci- 

 pital bones enter ; but they are also attached to the squamosals, 

 which may in some extinct forms overlap them externally so 

 much as almost to conceal them. This condition is an approxi- 

 mation to that of mammals in which the quadiate must be 

 regarded as being indistinguishably fused with the squamosal. 

 The nasal capsules remain largely cartilaginous and are covered 

 dorsally by the premaxillae, nasals and prefrontals. There is 

 always a prefrontal and a postfrontal, and usually a lacrymal. 

 The orbit is generally completed behind by the union of the post- 

 frontal with the jugals, between which a separate bone, the 

 postorbital, may be intercalated. The temporal fossa, which is 

 thus cut oft" from the orbit, is frequently divided into two by a 

 bridge of bone formed by the postfrontal sending backwards a 

 process to unite with an anteriorly directed process of the squa- 

 mosal. This bridge is called the supratemporal arcade and the 

 cavity between it and the skull the supratemporal fossa or 

 vacuity. Moreover the jugal is in many forms connected with 

 the lower end of the quadrate by a quadrato-jugal, which con- 

 stitutes the infratemporal arcade and forms the lower boundary 

 of what may be called the lateral temporal fossa or vacuity. In 

 Rhynchocephalia, Dinosauria, Crocodilia and Pterosauria, both 

 these arcades and both fossae are present ; in Ichthyosauria, 

 Plesiosauria, and Anomodontia, both arcades appear to be pre- 

 sent, but they are continuous and the lateral temporal fossa is 

 absent or very small (in some Anomodontia), so that there is only 

 one broad temporal arcade and one fossa (the supratemporal). 

 In Chelonia there is, as a rule, only one arcade, but it consists of 

 jugal and quadrato-jugal only and is the lower one, the supratem- 

 poral arcade not being developed (this is the mammalian arrange- 

 ment, save for the presence of a quadrato-jugal). In iYieLacer- 



