on the Fossil Genera Ichthyosaurus and Plesiosaurus. 



117 



the sketch in the margin ; but never having seen the part entire in any single 

 specimen, I have wished to keep this representation distinct from the dehnea- 

 tions of actual specimens which accompany this paper ; intending it only to serve 

 as an index to the detached bones represented in the Plates ; and offering it 



only as an attempt to restore the 

 part conjecturally. 



First we may notice the posterior 

 face of the os quadratum ; this bone 

 is well exhibited in the back of the 

 head represented fig. 8. PI. XVI. ; 

 and presents the outline given in the 

 annexed sketch. A sort of wing-shaped process (o') runs back from it to- 

 wards the inferior occipital (r) ; but this part is rather obscured by pyrites, 

 and it cannot be clearly ascertained whether this process may not be a se- 

 parate bone united to (o) by a suture. 



The inferior occipital (r), bearing the condyle, is frequently found in a de- 

 tached state : three figures of it are given in PI. XX. fig. 2. : — where a is a 

 view of the exterior surface ; 6 is a lateral view ; c is a view of the interior 

 surface. It is shown, beneath the temporal bone (here crushed in upon it) in 

 PI. XVI. fig. 8. ; and close to the posterior part of the pterygoids in the head 

 figured in PI. XVII. 



The superior (Q) and lateral occipitals (ss) I have only observed in one 

 specimen ; namely, the head which is figured in PI. XVII. They are there 

 slightly displaced, but may at once be recognised as surrounding the top and 

 side of the foramen magnum. 



The whole configuration of the occipital portion of the head, especially the 

 manner in which it is connected with the os quadratum, bears a much nearer 

 analogy to the crocodile than to the other lacertae. Some of the separate bones, 

 however, approach more nearly to those of the latter; the superior occipital, 

 for instance, in the crocodile, does. not extend low enough down to form the 

 upper margin of the foramen magnum ; but in the ichthyosaurus it appears to 

 do so, as in many lacertfe. 



IX. Palatal Bones, Pterygoids, &c. — These parts are exhibited in a very 

 satisfactory manner in the beautiful specimen figured in Plate XVII. 



This specimen seems to me clearly to prove that the arrangement of this 

 part agreed in all essential points with the crocodile, and differed from the 

 hicertsB ; as will appear by the following detail of circumstances : — 



