CROCODILIA. 109 



Most of the stages in the development and succession of the teeth of the Crocodiles 

 are described by Cuvier* with his wonted clearness and accuracy; but the mechanical 

 explanation of the expulsion of the old tooth, which Cuvier adopts from M. Tenon, is 

 opposed by the disproportion of the hard part of the new tooth to the vacuity in the 

 walls of the old one, and by the fact that the matter impressing — viz. the uncalcified 

 part of the tooth-matrix — is less dense than the part impressed. 



No sooner has the young tooth penetrated the interior of the old one, than another 

 germ begins to be developed from the angle between the base of the young tooth and 

 the inner alveolar process, or in the same relative position as that in which its imme- 

 diate predecessor began to rise, and the processes of succession and displacement 

 are carried on, uninterruptedly, throughout the long life of these cold-blooded 

 carnivorous reptiles. 



From the period of exclusion from the egg, the teeth of the crocodile succeed each 

 other in the vertical direction ; none are added from behind forwards, like the true 

 molars in Mammalia. It follows, therefore, that the number of the teeth of the cro- 

 codile is as great when it first sees the light as when it has acquired its full size ; and, 

 owing to the rapidity of the succession, the cavity at the base of the fully-formed 

 tooth is never consolidated. 



The fossil jaws of the extinct Crocodilians demonstrate that the same law regulated 

 the succession of the teeth, at the ancient epochs when those highly organized reptiles 

 prevailed in greatest numbers, and under the most varied generic and specific modi- 

 fications, as at the present period, when they are reduced to a single family, composed 

 of so few and slightly varied species, as to have constituted in the Systema Naturce of 

 Linnaeus, a small fraction of the genus Lacerta. 



Having completed the analysis of the constituent parts of the framework of the? 

 Crocodilia, which are petrifiable or conservable in a fossil state, and from the study 

 and comparison of which we have to gain our insight into the nature and affinities of 

 the extinct Reptiles, there remains only to be made a few observations on the charac- 

 teristic mode in which the bones are associated together in certain parts of the skeleton 

 in the present order, and especially in the skull. 



With regard to the trunk, the Crocodilia are distinguished from the Lacertilia and 

 from all other existing orders of Reptiles, by the articulation of the vertebral ribs 

 (pleurapophyses) in the cervical and anterior part of the dorsal segments by a head 

 and tubercle to a parapophysis and diapophysis. As this double joint is associated 

 with a double ventricle of the heart, and as the single articulation of every rib in other 

 Reptiles is associated with a single ventricle of the heart, we may infer a like difference 

 in the structure of the central organ of circulation in the extinct reptiles, manifesting 

 the above-defined modifications in the proximal joints of the ribs. 



* Op. cit.,pp. 90-3. 

 Q 



