484 BRITISH FOSSIL REPTILES. 



that it pushes away his temporal (Schiafenbein) from the orbit. In Pterosauria the post- 

 frontal (PI. 17, 12) is undoubtedly interposed between the bone I determine as 'mastoid' 

 (ib. S) and the orbit (ib. o) ; and my ' mastoid ' in Pterosauria answers to Cuvier's and 

 Hallnian's 'temporal,' i.e. squamosal, in Birds. We may conclude, therefore, that Von 

 Meyer's ' Schlafenbein ' in Pterosauria is that marked 8 in the skull of Pterodactylus 

 crassirostris} 



Certain it is that no bone answering to 8 in Pis. 1.5, 10, 17 of the present Work 

 contributes to the formation of the orbit in any Bird. In the great majority of that 

 class, as is well known, the rim of the orbit is incomplete below ; it is formed above 

 by the frontal, before by the prefrontal and lacrymal (' antorbital ' of ornithotomists), 

 behind by the postfrontal (' postorbital,' ib.). Where, as in some Psittacidce,^ the orbital 

 rim (' Augenhohlenrandes ') is complete, the lower complement is formed by an extension 

 of ossification from the antorbital to the postorbital processes, independently of either 

 Cuvier's temporal (s) or my squamosal (27) in Birds. 



I confess that the foregoing result of the analysis of a main ground of Von Meyer's 

 assertion as to the " incontestable similarity between the Pterosaurian and Avian types of 

 cranial structure " has not a little tended to shake my confidence in the grounds on which 

 he has pronounced definite judgment on the matter. So far as we have yet got evidence 

 of the structure of the skull in Pterosauria, it seems that, contrary to the rule in Birds, 

 the orbital rim is entire ; and that its lower border is completed by the zygomatic arch, and 

 chiefly, if not exclusively, by the malar element ; whereas, such arch passes freely beneath 

 the orbital rim in the few Birds with that rim entire. Now, in this part of the cranial 

 structure the Pterosauria agree with the Crocodilia : as in them the malar (26) sends up a 

 process to unite with one descending from the postfrontal (12) to complete the orbital 

 rim behind. 



In the small species of Pterodactyles (Pf. lonyirostris, Pt. scolopaciceps, and in the 

 perhaps immature animal represented by Pt. brevirostris) the hind convexity of the cranial 

 wall is not marked by the apophysiary developments of paroccipital and mastoid, and 

 accordingly resembles that part of the cranium in Birds, especially the smaller Grallce ; but 

 before this similarity of shape can be pressed into the argument for the Avian affinity of 

 the Pterosauria, it should be shown to be common to or constant in the extinct volant 

 order. 



But this is far from being the case. When a Pterosaur has gained the size of 

 Pterodactylus crassirostris ^ or Pter. suevicus^ the back of the skull shows no cerebral 

 swelling, but only the crests and processes for muscular attachments, as in other Reptilia 



1 Pterosauria, PI. 1, figs. 3 and 4. 



- 'On the Archetype and Homologies of the Vertebrate Skeleton,' Svo, 1848, pi. i, fig. 1 {Calyplo- 

 rJiynclms) ; 'Anatomy of Vertebrates,' Svo, vol. ii (1866), p. 51, fig. 30 {Psittacus), also p. 63. 

 ^ Goldfuss, op. cit., pi. vii. 

 Quenstedt, op. cit. 



