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284 Middle and Upper Triassic. Thalattosaurus Merriam, Nectosaurus Merriam, California.

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 foramen, tabulars, or [dermo]supraoccipitals, and doubtfully, [if] ever, the interparietals. Teeth thecodont, confined to jaws, rarely absent. Vertebrae never notochordal, nor the dorsal intercentra persistent.

From small to rather large, crawling or leaping reptiles, characterized 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. Mesopodials 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.

Typically a group of small, slender, climbing 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 epipodials 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