302 PEOF. H. G. SEELET ON SIMILITUDES OF 



dile-skull behind the orbit and between the postfrontal and malar 

 would need to be covered by two bones — a postorbital (completing 

 the orbit), and a supraquadrate (between the squamosal and 

 quadrato-jugal) . 



These changes would probably bring the quadrate bone vertical. 

 The parietal and frontal would both have to be double ; and the 

 anterior nares would have to be divided and carried backward 

 between the maxillary and nasal bones till they met the lachry- 

 mals, prolonging, with them, the premaxillary bones, partly at the 

 expense of the maxillary bones, and partly hiding them by overlap. 

 Then, by adding a foramen parietale between the parietal and 

 frontal bones, so far as the essential external characters went, 

 the head of a crocodile would have become the head of Ichthyo- 

 saurus. Then, to complete the correspondence on the palate, it 

 would be necessary to connect the quadrate bone made vertical 

 with the hinder angle of the pterygoid, and to separate the ptery- 

 goid and palatine bones so as to exhibit the basisphenoid and pre- 

 sphenoid, circumscribe a large pear-shaped palatal vacuity wide 

 behind, and obliterate the maxillo-pterygoid fossae by pressing the 

 palatine against the transverse bone. The vertical position of the 

 maxillaries draws them apart on the palate, and away from the 

 palatines, so that the premaxillaries are introduced internal to 

 the maxillaries in front ; and the vomers are introduced between 

 the premaxillaries and the diverging palatines behind. Thus by 

 opening the crocodilian palate it becomes ichthyosaurian. 



The resemblances between the two types are thus seen not to 

 be close ; but the differences are chiefly dependent upon the posi- 

 tion and condition of the orbits and nares. In both the occipital 

 condyle is single ; but in Crocodile it is hemispherical and small, 

 and its upper angles are mad6 by the exoccipital bones. In both 

 the temporal fossa is surrounded by parietal, squamosal, and post- 

 frontal bones. In the orbit the differences are that in Ichthyo- 

 saurus the frontal bone is entirely excluded, and the postorbital 

 bone becomes ossified. The lower jaw^ has a general resemblance 

 in both ; but the os articulare is longer in Crocodile than in Ich- 

 thyosaurus. The teeth placed in a groove are in this said to be 

 comparable to the posterior part of the jaw in the Black Alligator. 



In the vertebral column there is but little other resemblance 

 than that both have long tails. The chief points in which the 

 Crocodile differs are : — in having the vertebrae much longer and 

 less numerous, furnished with neural arches which unite sutu- 



