814 PKOF. H. G. SKELET ON SIMILITUDES OF 



§ 12. The Dinosaurian Affinities of Ichthyosaurus. 



The Scelidosaurus has the median roof-bones of the skull all 

 double ; the nasal bones are large and elongated, but they do 

 not extend so far back as in Ichthyosaurus, and so the frontal 

 bones occupy a much larger area of the skull. The frontal bone 

 similarly does not enter into the orbit ; but in Scelidosaurus the 

 superior border is made by an anterior division of the postfrontal, 

 which Prof. Owen names the superorbital bone. The orbits are 

 vertical in both, and the temporal fossae are horizontal. The bone 

 behind the orbit in Ichthyosaurs is named postorbital; and as 

 the sequence of the bones is the same in both, an advocate for 

 a uniform nomenclature might propose to name the dinosaurian 

 postfrontal postorbital, and the superorbital postfrontal. Then the 

 bones surrounding the orbit would be the same in both. In both 

 the eye is defended with sclerotic plates. But there is no supra- 

 quadrate in the Dinosaur, and no clear evidence of a quadrato- 

 jugal, while the quadrate would differ from that of Ichthyosaurus 

 in its slender form and in the long inner process which laps along 

 the pterygoid. 



The palate of a Dinosaur is not very like that of a lizard, the 

 pterygoid bones being more expanded ; the pterygoid bones in 

 Scelidosaurus are very unlike those of Ichthyosaurus. 



The teeth of Dinosaurs are chieily in tlie maxillary bone ; and 

 these teeth are always serrated. In Hypsilophodon the pre- 

 maxillary teeth are very different from the maxillary teeth, and 

 so are unlike those of Ichthyosaurus, although the crowns are 

 conical. 



The vertebral column has nothing in common, the dinosau- 

 rian centrum always being elongated as in Plesiosaurs, never deeply 

 cupped, without tubercles for ribs on the dorsal vertebrae, always 

 furnished with a large neural arch. In the caudal region there 

 are chevron bones and transverse processes. 



The pectoral girdle would appear to diflfer by the Dinosaurs 

 being devoid of clavicle and interelavicle, and showing traces of a 

 partly (?) osseous sternum. But of the other bones, the scapula 

 and coracoid have considerable resemblance of form. The coracoid 

 bone, for instance, usually referred to Iguanodon cannot be re- 

 garded as having any character to distinguish it from Ichthyosau- 

 rus ; and the scapula differs chiefly in its relatively greater 

 length, and the development of an acromial tubercle or prearti- 

 cular prolongation on the anterior margin. The pelvic arches 



