300 PBOF. H. G. SEELEl ON SIMILITUDES OE 



in birds than in Iclithyosaurs. The occipital condyle is similarly 

 single, but the exoccipital bones partly contribute to form it, iu 

 birds. In both types the base of the sphenoid is expanded, 

 and gives attachment in front to a long slender prsesphenoid bone. 



There is a difficulty in determining the bones of the Ichthyo- 

 saurian palate : the large bones which meet the quadrate bones 

 and lap round the sides of the basisphenoid are unlike in form 

 and relations to the style-shaped pterygoids of birds ; then there 

 is no certain evidence whether they give attachment to large and 

 more bird-like palatines (usually anchylosing with the ptery- 

 goids), or whether the entire bone is pterygoid bone. But in any 

 case there is a difference from birds in the long anterior ends 

 being prolonged between the vomers, and in the existence of 

 a long bone between the maxillaries, which, if the anterior parts 

 of the inner pair of bones are the palatines, would be transverse 

 bones, and Vv'hich, if the entire bone is the pterygoid, would be 

 palatine bones. 



In many birds there is similarly a circle of sclerotic bones to the 

 eye. Among the more striking differences, in birds the orbit is not 

 usually surrounded by a circle of separate bones. The prefrontal 

 and lachrymal are anchylosed together. The quadrate bone is 

 never hidden by other bones, though it is partially covered iu 

 some struthious birds ; there are no postorbital or supraquadrate 

 bones, and usually no postfcontal. The lower jaw in Ichthyosau- 

 rus, except the hoof-like articular bone, is made by a number of 

 long splint-like bones overlapping each other laterally, unlike 

 those in the jaw of a bird. 



There is nothing like the vertebral column of Ichthyosaurus 

 among birds. The chief differences are in the absence of separate 

 cervical and caudal ribs in the bird, in the large sacrum, in the 

 transverse platform-processes to the dorsal vertebrae, in the union 

 of the neural arch in each vertebra with its centrum, in the 

 elongated centrum of the bird (which, however, exceptionally has 

 biconcave articular ends, especially in the tail and back). There 

 is a great difference usually in number of vertebrae, especially as 

 seen in the relative lengths of neck and tail. 



The costal ribs of Ichthyosaurs have two heads ; but they ar- 

 ticulate not with facets (except the upper head in the neck) or 

 concavities, but with tubercles, which are always on the centrum. 

 The median sternal ribs of Ichthyosaurs have in birds become a 

 continuous median ossification or sternum. 



