254 FOSSIL REPTILES. 



The frontals cover but a small portion of the cerebral cavity, 

 for they are but short, and form^ in conjunction, a lozenge 

 more wide than long. 



The parietals form together a pentagon^ the most acute 

 angle of which unites to the occipital spine. They cover more 

 than half of the cerebral case, and go back by a squamous 

 suture on the occipital and os petrosum. On each side the 

 parietal descends very low into the temporal foss. It occupies 

 there almost all the space occupied in the crocodile by the 

 temporal wing of the sphenoid^ and there remains of this wing, 

 in the tortoise, only a very small piece, which unites itself on 

 one side to the descending portion of the parietal 3 on the 

 other, to the palatine, to the internal pterygoidean bone, to 

 the body of the sphenoid, to the os tympani, and to the os 

 petrosum. 



The jugal bone is, as usual, articulated with the external 

 and posterior angle of the maxillary. It is narrow, and ex- 

 tends under the orbit, behind which it meets the posterior 

 frontal which completes the frame in this part, and the squa- 

 mous temporal, which of itself forms all the zygomatic arch, 

 an arrangement of which there are a multitude of examples in 

 the cetacea. 



This temporal widens to unite itself to the os tympani, which 

 is extremely large. It forms almost a complete osseous frame 

 for a wide tympanum ; and under this it descends as an apo- 

 physis for the articulation of the lower jaw. This frame gives 

 entrance into a vast cavity, completed only at its upper angle 

 by the mastoidean bone. At the bottom of this cavity is a 

 foramen, through which the auditory osselet passes to arrive 

 at a second cavity, formed externally by the tympanic bone, 

 internally by the os petrosum and occipitals, underneath a little 

 by the sphenoid, and closed behind by cartilage. This tym- 

 panic bone, moreover, composes a good part of the posterior 

 parietes of the temporal fossa. 



Between it and the parietal in this same foss, the os petro- 



