AND AYES OF NORTH AMERICA. McMinn long longitudinal, posterior, supporting the pubis in front on a process. Ribs flee, double headed. Neural arches united by suture ; chevron bones present. DIXOSAURIA. Limbs ambulatory. External nostrils anterior. Inferior pelvic elements in contact transversely, acetabulum imperforate. Sacrinn of six vertebrae. Neural arches attached by suture. Premaxillary single or double ; Teeth wanting or represented by a pair of tusks, or canines. No COlUilleilft. ANOMODONTIA. Limbs ambulatory. External nostrils anterior. Inferior pelvic elements in contact transversely. Sacrum of two vertebrae. A columella. Clavicle, episternum and xiphisternum present, united. Chevron bones. • RHYNCHOCEPHALIA. 00 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, Thecodontia, and Crocodilia, appear to be related by a close bond, as 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 • of 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 maxillaries. When they bear teeth the latter form a series within and parallel to that Of the maxillary bone. This structure occurs in Sauropterygia, as Nothosaurus, (see fig. 4.) and in Rhynchoce-*See Huxley on relations of Plesiosaurus to Teleosaurus, Journ. Geol. Soc. Loud. smut!. PHILOS°. SOC.-VOL. XI V. 9
