ORGANIZATION OP THE OENITHOSAURIA. 95 



character is reptilian it must be a resemblance to either crocodiles, 

 lizards, or serpents, but can only be so regarded provided the 

 predominant resemblances of the skeleton prove to be with those 

 ordinal groups. 



The Ornithosaurian vertebras, however, show a resemblance to 

 those of birds and many mammals in the small size of the cen- 

 trum and in the large extent to which the sides of the neural arch 

 contribute to form the lateral parts of the intervertebral articula- 

 tion. I have not detected a like structural condition in the ver- 

 tebrae of reptiles. But the character becomes modified in impor- 

 tance by the relation being reversed in some mammals, since the 

 canal for the spinal cord is sometimes partly formed by the 

 centrum in Cetacea. 



The atlas and axis resemble those of a bird in proportion and 

 form ; but the condition of the odontoid process is different. When 

 the atlas separates from the axis of a bird, the odontoid process is 

 seen usually, if not always, to have formed the upper part of the 

 cup for the occipital condyle. In OrnithocJieirus a perfect disk 

 comes away from the axis, and displays a slight prominence on 

 the upper part of the anterior face of the axis with a concave space 

 around it. But I have no evidence whether the elevation represents 

 the centrum of the atlas in a diminished form, or whether that 

 bone unites with the other elements of the first vertebra, after the 

 pattern of Plesiosaurus and Ichthyosaurus, as would seem not 

 improbable. It thus differs alike from birds and reptiles. 



Von Meyer states that the dorsal vertebrae vary in number from 

 12 to 16 ; but in associated sets of bones from the Cambridge 

 Grreensand the dorsal vertebrae are few. They resemble those of 

 a bird in relative shortness, but do not appear to form a trans- 

 verse platform from which the neural spine rises, as do similar 

 vertebrae of adult birds and crocodiles, in this respect being more 

 like vertebrae of lizards. 



Sometimes the centrum is flat on the visceral side, as in Apteryx, 

 sometimes rounded, as in such birds as the Heron. Altogether 

 the dorsal region is less bird-like than is the neck, but the diver- 

 gences do not show marked resemblances to any existing ordinal 

 group of reptiles. 



The nature of the attachment of the ribs may perhaps be variable. 

 Several forms certainly possessed double-headed ribs like those 

 of birds and mammals ; and all specimens and figures, including 



LINN. JOUEN. — ZOOLOGY, TOL. XIII. 9 



