284 THE OSTEOLOGY OF THE REPTILES 



Middle and Upper Triassic. Thalattosaurus Merriam, Necto- 

 saurus Merriam, California. 



A A. SUPERORDER ARCHOSAURIA 



Dorsal ribs attached exclusively to the arch, at least anteriorly, by 

 two articulations, the cervicals to arch and centrum. Usually an 

 antorbital vacuity. The quadratojugal is well developed and usually 

 enters the border of the lateral temporal opening. No parietal fo- 

 ramen, tabulars, or [dermojsupraoccipitals, and doubtfully, [if] ever, 

 the interparietals. Teeth thecodont, confined to jaws, rarely absent. 

 Vertebrae never notochordal, nor the dorsal intercentra persistent. 



15. ORDER PARASUCHIA 



From small to rather large, crawling or leaping reptiles, character- 

 ized especially by the normal pelvis, absence of a secondary palate, 

 and a large antorbital opening. Body usually with dermal armor. 

 Roof bones of skull always paired; postfrontals present. Vertebrae 

 amphicoelous or platycoelous. Clavicles and interclavicle present, 

 the corocoid not elongate. Parasternal ribs generally present. Meso- 

 podials imperfectly known; phalanges not reduced. 



The Parasuchia in the present sense were long united with the 

 Crocodilia as two suborders, the Parasuchia, sens, str., and the 

 Pseudosuchia or Aetosauria, but the marked differences in skull and 

 pelvis justify their ordinal separation. By some authors the three 

 suborders here recognized are each given ordinal rank. Sclerotic 

 plates are known in a single genus of Pseudosuchia. 



A. Suborder Pseudosuchia 



Typically a group of small, slender, cHmbing or leaping reptiles 

 with more or less elongated hind legs. The external and internal 

 nares are near the extremity of the more or less pointed skull; the 

 lateral orbits are large, as are also the antorbital openings. The epi- 

 podials are long, the clavicles and interclavicle slender. 



None of the forms referred to this suborder is completely known, 

 and among the known forms there is a considerable diversity of 

 structure, some departing so widely, perhaps, that their location here 

 is provisional. Of the more typical, Scleromochlus has no dermal 



