LIASSIC FORMATIONS. 
107 
magnum in Crocodiles, but contributes, in Lizards, a share thereto, as in Ichthyosaurus. 
The paroccipital is confluent with the exoccipital in both Crocodiles and Lizards, as it is 
in Plesiosaurs d It remains distinct in Chelonians as in Ichthyosaurs. 
The extension of the mastoid upon the occipital region of the skull gives it an aspect 
of solidity more like that in Crocodiles than in Lizards ; but this is an adaptive conforma- 
tion, and depends on the need of an extent of bony surface for the implantation of the 
powerful nuchal muscles mainly concerned in wielding a head produced into long and 
heavy jaws, beset, as a rule, with formidable teeth ; it also relates to the stability of the 
prow of the Pish-lizard in cleaving the watery element. The occipital aspect of the 
diverging extensions of the parietals, and the presence of the ‘ fontanelle,’ called ' foramen 
parietale ’ in Plesiosaurs ^ as well as Ichthyosaurs, are more decisive instances of the closer 
affinity of Lizards, than of Crocodiles, to the antecedent marine types of Beptilia. No 
part of the parietal extends upon the occiput in Crocodiles, but this is the case in 
Plesiosaurs as in Ichthyosaurs. 
In adaptive relation to the mandible and its armature I regard the relative size and 
shape of the tympanic, the number of bones amongst which it is wedged, and the double 
buttresses extended on each side from the facial to the cranial part of the skull. In these 
characters the Ichthyosaurs resemble the Crocodiles ; but the upper or postfronto- 
mastoid zygoma and the lower malo-zygornatic one are present in some extinct as well 
as existing Lizards, e. g. Bhynchosaurus * and the Bhynchocephalia* 
In the exclusion of the mid-frontal from the orbit Ichthyosaurus differs from the 
Crocodiles and from most Lizards, but it is in the Lacertian order only that exceptions 
occur of repetitions of this Ichthyosaurian structure.® In the position, construction, and 
parial character of the external nostrils the Lizards repeat the Ichthyosaurian and Plesio- 
saurian type, from which the Crocodiles have departed, but the lacrymal is excluded from 
the formation of the nostril in all Lizards. In the small relative size of the maxillaries, 
especially as compared with the premaxillaries. Ichthyosaurus differs from both Plesio- 
saurs and Crocodiles, and still more from Lizards : here we have in Pishes the nearest 
resemblance to the subjects of the present Monograph. Nevertheless, as in Lacertilia, 
the anterior boundary of the external nostril is formed by the premaxillary ; and, as the 
marine Beptilia, like the marine Mammalia, needed to have the nostrils at or near to the 
upper part of the head, so, agreeably with the Lacertian type, the premaxillaries, how- 
ever they might be produced forward, retain in Ichthyosaurus, as in Plesiosaurus, the 
posterior relations with their antorbital nostrils. 
1 ‘ Monograph on the Sauropterygia,’ Palseontographical Vol. for 1863 (1865), p. 8, pi. iii, fig. 1, 2—4. 
2 Ib., pL xvi, fig. 1, 7 , 
* ‘Trans. Cambridge Philos. Society,’ vol. vii, 4to (1842), p. 350, pi. v. 
^ ‘ Catal, of Osteological Series in Mus. Coll. Surgeons,’ 4to, 1853, p. 143, No. 663; and Gunthee 
‘Phil. Trans.,’ mdccclxvii, p. 32. 
^ In Chameleo parsoni, e.g. Cdvier, ‘ Oss. Foss.,’ v, pt, ii, pi. xv, fig. 80. 
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