34 Mr. H. G. Seeley on the Structure of 



contributes the anterior boundary to the articular surface for 

 the quadrate. The articular bone in its anterior part rests 

 upon the angular bone, but behind the articulation the speci-> 

 men is fractured. In passing backward the depth of the jaw 

 becomes much less as it nears the articulation ; here the bone 

 widens and extends inward precisely as in birds : the heel 

 behind the articulation is of varying length and form. 



In every respect this structure is like that of a bird*, if we 

 except the want of evidence of the existence of the opercular 

 bone ; but as it is possible that the interior surface which I 

 have attributed to the dentary bone may be for the opercular 

 bone, the correspondence may be more perfect than I have 

 supposed it to be. If there were only four elements in the 

 lower jaw, the whole arrangement would be very like that 

 seen in turtles. 



If, now, we endeavour to form a conception of the Ornitho- 

 cheiroid head in its structural resemblances to other animals, 

 we see that the entire skull, so far as known, is formed after 

 the manner of birds in every region, except in the malar, 

 quadrato-jugal, and postfrontal bones, which, though of the 

 reptile type, are not similarly placed in any reptile, and must 

 therefore be regarded as an Ornithosaurian modification of 

 tlie bird's skull. The lower jaw may be Avian or Chelonian. 

 The teeth must be regarded as Ornithosaurian, curiously com- 

 bining Reptilian and Mammalian characters. 



The points in which the Cambridge head certainly differs 

 from other types are not important. They consist, if my iden- 

 tification is right, in the brain being closed by a bony mass in 

 front, which extends forward partly between the orbits. This 

 structm-e has not been figm-ed in any of the true Pterodacty- 

 lida3, and does not appear to be constant in the Ramphorhyn- 

 chidge, and seemingly is equally inconstant in Cambridge 

 genera. But in the one specimen in which such a mass 

 occurs it is very wide from side to side, is anchylosed with 

 what I interpret as the fore part of the sphenoid, and furnishes 

 the authority for the convex mammal-like under part of the 

 brain ; and the bone also resembles the preorbital part of the 

 ethmoid in the duck and in many birds. This resemblance 

 is, indeed, so close that, but for the detailed correspondence 

 of the base of this fossil specimen (Ornithosauria, pi. 11. 

 fig. 7) to the base of the sphenoid in the back of an Omitho- 



* In ' The Omithosaima ' it is stated (p. 92) that the six elements of 

 the lower jaw may be comited on each side. It would have been more 

 accm-ate to have said five ; for the separation of the coronoid from the 

 articular is not well made out. 



