176 PALEONTOLOGY 



"squamosal." Above 27, between it and 8, is the "supra- 

 squamosal." Essentially it indicates the tendency to excessive 

 dermal ossification of the skull, like that which extends into 

 the superficial temporal fascia from the squamosal and mastoid 

 in the Chelonia ; this separate ossification in Arclicgosaurus 

 roofs over the temporal fossa. It is the homologue of the 

 supernumerary surface-bone called " supersquamosal " in the 

 Labyrinthodonts ; and both this and the "postorbital" corre- 

 spond in position with the posterior suborbital scale-bones in 

 Amia and Lcpidostcus. 



The hinder angles of the skull are formed by the tympanic ; 

 in young individuals the tympanic does not extend backward 

 beyond the par-occipital, but as age advances it projects further 

 backward. It appears to abut internally against the pterygoid. 



The two rami of the mandible were loosely united at a 

 short symphysis, not exceeding the breadth or depth of the 

 jaw at that point ; the depth gradually augments to near the 

 articular end, but never exceeds a sixth, and is usually only 

 an eighth of the length of the jaw, no definite coronoid process 

 being developed ; the upper and lower borders are nearly 

 straight as far as the deepest part. The lower border behind 

 this part rises rather abruptly to an angle, which is just below 

 the articular pit. The angular element (30) presents a con- 

 vexity answering to the point of ossification whence some faint 

 ridges radiate upon its outer surface. The dentary (32), if it 

 does not form the articular surface, begins very near it, and 

 each ramus appears to be composed of these two bones. The 

 dentary developes the coronoid rising. Neither articular nor 

 splenial element has been clearly demonstrated. If an articu- 

 lar element has existed, it has been very small. 



From fishes the lower jaw of Arclicgosaurus differs in the 

 great length or forward extension of the angular piece (30) ; 

 but it resembles the piscine type in the simplicity of its com- 

 position. The angular piece is, however, longer in the Ganoids 



