on the Fossil Genera Ichthyosaurus and Plesiosaurus. 109 



viously impossible to obtain from the examination of a few individuals a cer- 

 tain determination of the size of the species when full grown. 



III. Structure of the lower Jaw. — In order to indicate more clearly the 

 analogies of the ichthyosaurus in this part of its structure^ I shall compare each 

 of the bones of its lower jaw with the corresponding- bones both of the cro- 

 codile and of the other lacertae. ^ 



1. The Dental bone. — The furrow which contains the teeth, exhibiting in 

 the slight ridges that traverse it the rudiments of a division into distinct alve- 

 oli, approximates (as has been seen) much more nearly to the analogous part 

 in the crocodile, than to that in the other lacertae. See fig. 12. PI. XV. 



Another circumstance, however, has an opposite bearing: namely, the distri- 

 bution of the holes on the outside of its anterior portion, which convey the 

 branches of the inferior maxillary nerve and vessels. These in the crocodile 

 exhibit a series of small dots irregularly dispersed ; in the other lacertae and 

 in the ichthyosaurus they appear as a regular row of conspicuous perforations. 

 See fig. 12. PI. XV. and fig. 6. PI. XVI. 



The ichthyosaurus has eight or nine of these perforations, branching out 

 laterally from the canal which runs longitudinally through the substance of the 

 dental bone. The posterior branches are inflected backwards, the anterior 

 forwards. As these branches are sent out from the central canal to the exte- 

 rior, so there are others directed to the interior, to supply the teeth. Fig. 12. 

 PI. XV. exhibits a portion of the dental bone, partly cut away, in order to 

 expose this central canal and its branches. The same thing is also shown in 

 PI. XVI. fig. 6. — and in the section, fig. 1. 



2. The Angular bone. S. The Coronoid bone, and 4. The Crescent-shaped 

 bone. — The structure of all these bones is essentially connected with the 

 disposition of the muscles moving the lower jaw ; namely, the temporal^ the 

 masseter, and the pterygoid muscles. The crocodile in these respects exhi- 

 bits a peculiar conformation ; of which a large oval hole, placed at the junction 

 of the dental angular and coronoid bones, affords a strong osteological indi- 

 cation. The organization to which it belongs is as follows : — In many species 

 of crocodile the temporal muscle (the grand motor of the lower jaw in most 

 animals) is scarcely more than a rudiment : and although in the gavial, where 

 the great length of the lever to be acted upon requires an increase of power, 

 this muscle is more developed ; still, even here it is but imperfectly displayed. 

 In order to compensate for this deficiency, the masseter, on which the addi- 

 tional functions usually belonging to the temporal muscle thus devolve, re- 



