LIASSIC ICHTHYOSAURS. 65 



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 post- 

 fronto-mastoid zygoma and the lower malo-zygomatic one are present in some extinct as 

 well as existing Lizards, e.g. Rhi/nchosaurus^ and the Rlii/nchocephalia? 



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 Pleiso- 

 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 maxil- 

 laries, especially as compared with the premaxillaries, Ichihi/osaurus differs from both 

 Plesiosaurs and Crocodiles, and still more from Lizards : here we have in Fishes the 

 nearest resemblance to the subjects of the present Chapter. Nevertheless, as 

 in Lecertilia, the anterior boundary of the external nostril is formed by the pre- 

 maxillary ; 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, however they might be produced forward, retain in Ichthyosaurus, as 

 in Plesiosaurns, the posterior relations with their antorbital nostrils. 



Both Lacertians and Crocodilians differ from Ichthyosaurs in the connections of 

 the nasal with the maxillary. The Crocodiles resemble them in the inter-pre- 

 maxillary suture ; its presence is an exception in Lacertians, the Rhynchosaurians and 

 Rhynchocephalians* again affording such examples; I have found it obliterated in a 

 Plesiosaurus dolichodeirus} In the position and formation of the palatal nostrils the 

 Lacertians agree with, whilst the Crocodiles widely depart from, the Ichthyosaurian 

 type. The apertures are distinct or parial in the Plesiosaurs, but are placed far back." 



In the structure of the mandible the dentary resembles that element in Lizards, and 

 differs from that in Crocodiles, in being pierced externally by a longitudinal series of 

 nervovascular foramina ; it differs, also, from the dentary in Crocodiles in its posterior 

 termination being above instead of beneath the fore end of the surangular. In the 

 aniphicoelian Crocodiles the vacuity between the angular and surangular is much 

 reduced in size; it is still smaller in Rhynchocephalians ; it is absent in Ichthyosaurs, as 

 in Plesiosaurs' and most Lizards. 



In the conformation of the posterior angle and the robustness of the articular 



1 '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 Guntheb. 

 ' Phil. Trans.,' mdccclsvii, p. 32. 



^ In CJiameleo parsoni, e.y. Ccvier, ' Oss. Foss.,' v, pt. ii, pi. xv, fig. 80. 



* Pi. xvi, fig. 1, 23. "Oil the Affinities of Rhynchosaurus," 'Annals and Mag. of Nat. History,' iv, 

 1859, p. 237. 



6 PI. iii, fig. 1. 6 PI. xvi, fig. 2, r, r. 7 pi, ji. 



9 



