AND AVES OF NORTH AMERICA. 



33 



Ischium long Longitudinal, posterior, supporting the pubis in front on a process. 



Ribs free, double; headed. 



Neural arches united by suture; chevron bones present. 



D1NOSAURIA. 



Limbs ambulatory. 



Externa] nostrils anterior. 



Inferior pelvic elements in contact transversely, acetabulum imperforate. 



Sacrum of six vertebra?. 



Neural arches attached by suture. 



Premaxillary single or double; 



Teeth wanting or represented by a pair of tusks, or canines. 



No columella. anomouontia. 



Limbs ambulatory. 



External nostrils anterior. 



Inferior pelvic elements in contact transversely. 



•Sacrum of two vertebrse. 



A columella. 



Clavicle, episternum and xiphisternum present, united. 



Chevron bones. RHYNCHOCEPHALIA. 



The important modification in the mode of articulation of the quadrate bone, which 

 characterizes this order has been, overlooked in most of the systematic arrangements of 

 the extinct and living Reptilia. The subordinate forms differ in important points, but the 

 groups Sauropterygia, Theeodontia, and Crocodilia, appear to be related by a close bond, 

 :|S for example the marine, the terrestrial, the Sphargid, and the Pleurodire tortoises. The 

 extremities are modified for all modes of progression, except that of flight, in both; while 

 as much gradation between these types is seen in one as in the other. In the characters 

 01 the anterior and posterior nasal openings, there is a great range in these types, but 

 the transitions in these respects occur successively from Crocodilus to Teleosaurus,* to 

 Belodon, to Plesiosaurus and Nothosaurus. 



An important definitive character is found in several types of the Archosauria. The 

 pterygoid bones are prolonged anteriorly between the palatines, and frequently as far as 

 the vomer, completely separating the palatines. The latter then lie exterior to the 

 pterygoids and between them and the maxillari.es. When they bear teeth the latter 

 form a series within and parallel to that of the maxillary hone. 



Tins structure occurs in Sauropterygia, as Nothosaurus, (see fig. 4,) and in Rhynchoce- 

 *See Huxley on relations of Plesiosaurus to Teleosaurus, Journ. Geol. Soc. Loud. 



AMERI. PHILOSO. SOO VOL. XIV. 9 



