301 PEOr. H. G. SEELET ON SIMILITUDES OJ? 



and an approximation of the prefrontal and postfrontal bones 

 together, so as almost to meet in Chelonians as they do in Ich- 

 thyosaurs. Of all that part of the ichthyosaurian skull which is 

 in front of the nares, the Chelonian skull has no representative. 

 If, then, we took such a truncated ichthyosaurian skull and tried 

 to approximate it towards a Tortoise, it would be necessary first 

 to remove the nasal bones entirely. This would expose much of 

 the frontal bones which they cover, and allow the prefrontal bones 

 to be squeezed together to meet mesially and make the upper 

 margin of the nares. A similar compression together of the ter- 

 minal minute remnants of the premaxillaries would make the lower 

 margin of a single terminal nostril like that of Testudo. The 

 foramen parietale must be obliterated, and the supraoccipital 

 developed and prolonged with the adjacent angle of the parietal 

 bones into a strong median occipital crest. The postorbital and 

 supraquadrate bones would be obliterated, and the malar, post- 

 frontal, and quadrato-jugal bones prolonged behind the orbit to 

 meet in a triradiate union. Then, with an inflating auditory ex- 

 cavation of the quadrate and squamosal bones, to outward view 

 the upper part of the Ichthyosaur's skull would have become 

 Chelonian. In the palate there is a primary difficulty with the 

 homology of the bones, because on the ichthyosaurian palate 

 there are small vacuities under the place of the external nares, 

 which might be regarded as posterior nares, either potential or 

 actual ; and they are surrounded chiefly by two bones, the vomer 

 internally and what might be the palatine externally, the pre- 

 maxillary sometimes entering in front. I adopted another inter- 

 pretation to explain the relation of the palate to that of Croco- 

 diles ; this interpretation would be as necessary to harmonize 

 it with that of Chelonians. Then, to complete the resemblance 

 to Chelonians, it would be necessary to unite the two vomers 

 into a single median vomer, into which the pterygoids should 

 not penetrate posteriorly ; and then, by extending the pterygoid 

 bones internally till they met mesially so as to hide the presphe- 

 noid and cover the palatal vacuity, the change would be perfect 

 but for the want of teeth. 



Thus the cranial resemblances of Chelonians to Ichthyosaurs 

 are so slight that they are scarcely of any value. The back of 

 the head is as unlike as the other parts : there the exoccipitals 

 contribute equally with the basioccipital to the condyle, and 

 in Testudiuates the opisthotic meets the squamosal bone only, 



