LIASSIC FORMATIONS. 107 



magnum in Crocodiles, but contributes, in Lizards, a share thereto, as in Ichthyosaurus. 

 The paroccipital is confluent with the exoGcipital in both Crocodiles and Lizards, as it is 

 in Plesiosaurs. 1 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 Fish-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 3 as well as Ichthyosaurs, are more decisive instances of the closer 

 affinity of Lizards, than of Crocodiles, to the antecedent marine types of Reptilia. 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-zygomatic one are present in some extinct as well 

 as existing Lizards, e. g. Rhynchosaurus 8 and the Rhynchocephalia. 4, 



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. 5 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 Fishes 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 Reptilia, 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 ' ?»Ionograph on (he Sauropteryffia,' Palseontograpliical Vol. for 1863 (1865), p. 8, pi. iii, fig. 1, 2—4. 



2 lb., pi. xvi, fig. 1, 7 . 



8 'Trans. Cambridge Philos. Society,' vol. vii, 4to (1842), p. 350, pi. v. 



4 'Catal. of Osteological Series in Mus. Coll. Surgeons,' 4to, 1853, p. 143, No. 663; and Gunther 

 'Phil. Trans.,' mdccclxvii, p. 32. 



5 In Chameleo parsoni, e.g. Cuviek, ' Oss. Foss.,' v, pt. ii, pi. xv, fig. 80. 



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