240 AMERICAN TESTUDINATA. Part II. 



Ichtliyosaun, the Plesiosauri, the Pterodactyl!, the Dinosauri, ete^ as constituting 

 several additional orders, these groups, as zoological divisions, have in themselves 

 the character of orders, that is to say, they exhibit, when compared with one 

 another, various degrees of complication of their structure, and stand, with refer- 

 ence to one another, higher or lower. It cannot be doubted, for instance, that 

 compared with Lizards, the Snakes are an inferior group, and that the Chelonians, 

 in which the different regions of the body are so distinctly marked and in which 

 the head for the first time acquires a greater movability upon the neck, stand 

 above the others, approaching indeed, in many respects, the class of Birds, especially 

 the lower families of aquatic Birds, both in their form and in their mode of 

 existence. 



Now, this gradation, acknowledged by all, inasmuch as all herpetologists place 

 the Chelonians at the head of this class and next to them the Saurians, while 

 the Opliidians occupy a lower position, will serve as an illustration of my definition 

 of orders as natural groups, characterized by the different degrees of complication of 

 the special structure of their class, which complications determine their relative rank 

 or standing. I would not, however, in this connection forget that some naturalists, 

 Strauss^ among others, have of late considered the Chelonians as a distinct class, 

 and not as an order among Reptiles. Now, let us apply the test of our rules 

 to this suggestion, remembering here again that these rules have been drawn froan 

 those classes of the animal kingdom, such as the Ecliinoderms, Acalephs, and Polyps, 

 in which the orders are still more distinctly marked out ua nature than in the 

 one now under consideration. 



To constitute a class apart from Ophidians and Saurians, the structure of 

 Chelonians ought to be built up in a different way and with different means from 

 that of Saurians and Ophidians. And now, is this the case ? The Chelonians, 

 like Saurians and Ophidiaas, undergo a development so identical, that we need 

 only compai'e the investigatioas of Rathke upon that subject with those contained 

 in this volume, to settle any doubts on that point. And as to structure, what 

 difference is there, excejit differences in complication of structure, between Ophidians, 

 Saurians, and Chelonians, both in their nervous systems and organs of senses, 

 in their locomotive apparatus and in their intestines? Is not even the skeleton 

 truly homologicaj in all of them?^ We cannot fail, therefore, to consider the 

 view as fully sustained, that Chelonians represent an order, and nothing but an 

 order, in the class of true Reptiles. 



' Strauss-Durkueiji, (II.,) Theologie de la N.v ami Ophidians, and that the position of theii- limbs 



ture, Paris, 1852, 3 vols. 8vo. ; vol. 1, p.. 99 and 398. and the frame of their shield does not place them 



^ For further evidence that the structure of tlie in an exceptional position, with reference to the 



Chelonians is truly homological with that of Saurians other Reptiles, see below, Sect. 6 of this chapter. 



