FOSSIL REPTILES. 255 



sum shows itself, and the cranium is closed behind by the oc- 

 cipital, which is here divided into six bones, and not into four; 

 for the lateral occipitals are each divided into two parts, the 

 external one of which the Baron terms the external occipital. 



The fenestra ovalis is situated, as in the crocodile, at the 

 OS petrosum and the common lateral occipital ; but the fenes- 

 tra rotunda is pierced in the external occipital, in the same 

 manner as it is in the lateral occipital of the crocodile. 



In this tortoise, as well as in the crocodile, the grand fora- 

 men for the passage of the fifth pair is in front of the os petro- 

 sum, between it and the temporal wing. In the sea-tortoise 

 this foramen is between the os petrosum, and the descending 

 portion of the parietal. 



The auditory osselet is simple, as in the crocodile, and 

 formed of a slender stem, which widens at its approach to the 

 fenestra ovalis, and attaches itself there by a round and con- 

 cave face, so that it has pretty nearly the figure of a trumpet. 

 The external edge of this stem is in a great part cartilagi- 

 nous, and is terminated by a plate of the same substance, and 

 of a lenticular form, which is enchased in the membrana tym- 

 pani, and which, perhaps, may be considered as analogous to 

 the malleus. 



The Eustachian tube is altogether cartilaginous or mem- 

 branous. It commences in the external chamber of the ca- 

 vity in the upper part, by a wide emargination of the posterior 

 edge of the tympanic bone, and goes obliquely within, passing 

 between that bone and the depressor muscle of the lower jaw, 

 as far as an emargination of the lateral and posterior edge of 

 the pterygoidean bone, through which it penetrates into the 

 back part of the mouth, on the side very near the articulation 

 of the lower jaw, and especially very far behind the interior 

 nostrils. 



The orifices of the two Eustachian tubes may be seen at the 

 palate, or rather at the back of the roof of the hinder mouth, in 

 the form of two little holes sufficiently remote from each other. 



