GENERAL STRUCTURE. IO51 



The same numbers obtain in some Dinosauria ; but in existing 

 Crocodilia, in which the fifth digit of the pes is aborted, the 

 numbers are 2, 3, 4, 4, 3 and 2, 3, 4, 4. Among the Sauro- 

 pterygia and in the Ichthyopterygia the number of phalangeals is 

 greatly increased. In a large number of forms the tail is long ; 

 but it is generally short in the Anomodontia, Sauropterygia, and 

 Chelonia. 



As regards the classification of Reptiles, scarcely any two writers 

 asree as to the number of orders into which the class should be 



o 



divided, and still less as to their mutual relations, and the larger 

 groups under which these orders may be arranged. There is indeed 

 but little difficulty in regard to existing forms, in which the few 

 orders have become more or less sharply differentiated ; but when 

 we go back to the early part of the Mesozoic epoch, we find that 

 nearly all the orders into which the class has been divided show 

 such signs of passing more or less completely into one another, that 

 it is quite impossible to exhibit their true relationship by any system 

 of linear classification. The best arrangement seems, therefore, to 

 group the orders under a series of diverging branches, which will 

 approximate to one another more and more as we recede in time — 

 until, about the epoch of the Lower Permian or possibly the Carbon- 

 iferous, it is probable that, if we knew all the extinct forms, these 

 branches would be seen to originate either from one, or from but very 

 few parent stems. In regard to the number of these branches, there 

 is still room for a considerable amount of discussion ; almost the 

 only absolutely sure ground that we can feel being the association of 

 the three orders forming the xArchosaurian branch. To a less extent 

 the same remark applies to the orders themselves ; and the right to 

 ordinal distinction of the Proterosauria is not admitted by many 

 writers, while there is not perfect accord in regard to that of the 

 Rhynchocephalia. The provisional arrangement which is adopted 

 in this work is a modification of one recently proposed by Dr G. 

 Baur, of New Haven, and may be tabulated as follows, viz. : — 



Theromorous Branch. Order 1. Anomodontia. 



Synaptosaurian n -J 



Streptostylic u X 



Archosaurian w 



11 2. 



Sauropterygia. 



" 3- 



Chelonia. 



, 4. 



Ichthyopterygia. 



■ 5- 



Proterosauria. 



, 6. 



Rhynchocephalia 



„ 7. 



Squamata. 



-, 8. 



Dinosauria. 



-. 9. 



Crocodilia. 



11 10. 



Ornithosauria. 



It may be added that the close approximation to the Amphibia 

 presented by the earlier members of several of these branches 



