LIASSIC FORMATIONS. 
59 
The divergence of the hinder prongs of the dentary exposes a small part of the splenial 
(30- The vacuity, if it he natural and not due to abrasion of a thin outer wall, is a long 
and narrow oval, 1 inch 8 lines in length, 6 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 Pterosauria. In 
relative size, as in the expanse of the antorbital vacuity, Pterodactylus crassirostris ^ 
comes nearest to Pimorp/iodon ; but the orbit is relatively larger, and the nostril much 
smaller. In Phamphorhynchus Gemmmyi 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. In Pterodactylus lonyir Osiris'^ the nostril is larger than the orbit; the 
antorbital vacuity is not half the size of the orbit. In Pterodactylus suevicus ® the antorbital 
vacuity is still smaller. In Pterodactylus Kochii ^ that vacuity is limited, as in Chlamydo- 
saurus, to the upper part of the boundary between the large orbit and the long and large 
nostril. In Pterodactylus lonyicollum ^ 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 Pterodactylus scolopaciceps and Pt. lonyirostris to the shorter 
and deeper cone indicated by Pt. conirostris^ and to the inflated and more or less 
anteriorly obtuse form exhibited by Bimorphodon and the more gigantic Pterodactylus 
simus? 
The position of the tympanic pedicle varies from the almost vertical one in Bimorpho- 
don to the almost horizontal one in Pterodactylus lonyirostris 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 Bimorphodon. The depth of the rami 
decreases behind the dentigerous part in Pterodactylus lonyirostris. 
The generic dental character of Bimorphodon has been given in detail in the special 
descriptions of the specimens figured in Pis. XVII and XVIII. The range of variety mani- 
1 ‘Monograph on Fossil Keptilia of the Cretaceous Formations’ (1851), Pterosauria, PI. XXVII, 
figs. 2 — 4. 
2 Ib., ib., fig. 1. 3 QuENSTEDT, Op. cit. 
^ Von Meyer, op. cit., tab. i, fig. 2. ^ Ib., ib., tab. vii, figs. 1 — 4. 
® Dixon’s ‘ Geology and Fossils of the Tertiary and Cretaceous Formations of Sussex, 4to, 184*6, 
PI. 38. 
^ ‘Monograph on Fossil Reptilia of the Cretaceous Formations’ {Pterosauria), Suppl. No. 3 (1861), 
PI. I, figs. 1 — 3. 
