Yol. 69, ,] SKELETON OP ORXITHODESMUS LATIDENSi 399 



The Orbit. 



In Dimorphodon and Scaphoyaathus the orbit is in front of the 

 articulation of the quadrate with the mandible, and in Pterodac- 

 tylus, Ptenodracon (Lydekker), Rhamphorhynchus, and Nyctosaurus 

 the orbit is above that articulation. It is posterior in Omitho- 

 stoma, but in Ornithodesmus it is, relatively, more so. In the 

 latter the orbit is widely removed from the anterior nares, which 

 in Ptenodracon and Omithostoma are in close proximity, much 

 more separated than in Pterodactylus, and still more than in 

 Rhamphorhynchus, Scaphognaihus, and Dimorphodon. A great 

 peculiarity is the extraordinarily-small moiety of the orbit formed 

 by the jugal, and also that this should be in the anterior margin 

 •only — not below and behind, as would have been supposed by 

 analogy with other species. So far as I am aware, there is no 

 skull recent or fossil which has anything conforming to this. In 

 1870 Owen 1 remarked on the complete orbital rim, and that the 

 lower border was mainly formed by the jugal. In Ornithodesmus 

 the rays or outrunners thrown out by the jugal and quadrato- 

 jugal over the infra-orbital vacuity apparently represent a vestige 

 of a once complete orbit. The reduction in size and the alteration 

 of the position of these bones have been caused by the elongation 

 of the muzzle drawing these bones forward. In Scaphognathus 

 purdoni the orbital fossa is the largest aperture in the skull, and 

 the bones are arranged according to the plan of Dimorphodon. 

 There is no closing-in of the bones that form their boundaries, but 

 the modification which eventually produced this effect in Oniitho- 

 desmus was in progress. 



The Supra-Temporal Vacuity. 



The supra-temporal vacuity is greater in depth and more ex- 

 ternally open even than in Scaphognathus, where it is deeper than 

 in any other genus. 



The Infra-Temporal Vacuity. 



The extension of the infra-temporal vacuity, both anterior and 

 posterior to the orbit, is another exceptional character. The 

 nearest approach is found in Pterodactylus antlquus, where it lies 

 obliquely below the hinder half of the orbit. It is interesting to 

 note that it occurs before and beneath, but not behind, in the 

 Dinosaur Diplodocus. 



The Supra- and Infra-Temporal Arcades. 



The exclusion of the jugal from the upper temporal arcade, the 

 •extension of the quadrate to the maxilla, and the squeezing-out of 



1 E.Owen, 'Foss. Eept. Liass. Form.' (Monogr. Pal. Soc.) 1870, pt. 3, 

 T- 62. 



