LIASSIC PTERODACTYLES. 481 



The divergence of the hinder prongs of the dentary exposes a small part of the splenial 

 (31). The vacuity, if it be natural and not due to abrasion of a thin outer wall, is a long 

 and narrow oval, 1 inch 8 lines in length, G lines in breadth. It is circumscribed behind 

 by the confluent angular and surangular elements (29). The angular (30) forms a slight 

 projection behind the articular concavity ; it expands vertically, and contracts transversely 

 as it advances, contributing a small share to the lower border of the vacuity, and con- 

 tracting to a point below the dentary, about 5 inches from the angular process. 



The range of variety shown by the skull is considerable in the order Pferosauria. In 

 relative size, as in the expanse of the antorbital vacuity, Ptcrodacti/lus crassirostris^ 

 comes nearest to Bimorphodoii ; but the orbit is relatively larger, and the nostril much 

 smaller. In Bhamphorhi/nchus Gemmiriffi the nostril and antorbital vacuity are of equal size, 

 and each is about one eighth the size of the orbit, which is proportionally larger than in 

 Dimorphodon. \a. Fterodadyliis longirostris" the nostril is larger than the orbit; the 

 antorbital vacuity is not half the size of the orbit. In Pterodacfi/Ius suevicus ^ the antorbital 

 vacuity is still smaller. In Pterodacti/his Kochii * that vacuity is limited, as in Cldamydo- 

 saurus, to the upper part of the boundary between the large orbit and the long and large 

 nostril. In Pterodacfi/Ius longicollmn ^ it appears to be wanting. 



The shape of the skull offers many modifications in the several species, from the long 

 and slender type of that of Pterodacti/lus scolopaciceps and Pt. longirostris to the shorter 

 and deeper cone indicated by Pt. conirostris,^ and to the inflated and more or less 

 anteriorly obtuse form exhibited by Dimorjihodon and the more gigantic Pterodactylus 

 simus? 



The position of the tympanic pedicle varies from the almost vertical one in Dimorpho- 

 don to the almost horizontal one in Plerodadylus longirostris and Pt. Kochii. In Pt. 

 crassirostris it shows an intermediate slope or position. 



The mandible, conforming in relative depth and length to the general shape of the 

 skull, has the symphysis longest in those species with long and slender jaws. In 

 Pterodactylus suevicus the symphysis extends along the anterior third part of the mandible. 

 In Pt. crassirostris it is shorter, and still shorter in Dimorphodon. The depth of the rami 

 decreases behind the dentigerous part in Pterodactylus longirostris. 



The generic dental character of Dimorphodon has been given in detail in the special 

 descriptions of the specimens figured in Pis. 15 and 16. The range of variety mani- 



1 Pterosauriu, PL 1, figs. 2 — -4. 



2 lb., fig. 1. 



^ QUENSTEDT, Op. cit. 



* Von Meyer, op. cit., tab. i, fig. 2. 

 ^ lb., ib., tab. vii, figs. 1 — 4. 



^ Dixon's 'Geology and Fossils of the Tertiai-y and Cretaceous Formations of Sussex, 4to, 1846, 

 PI. 38. 



' Pterosauria, PI. 6, figs. 1 — 3. 



