0. C. Marsh — Omithopoda of the American Jurassic. 89 



Laosaurus. 



Premaxillaries edentulous. Teeth small and irregular. Cer- 

 vicals short and flat. Six vertebrae in sacrum ; no peg and 

 notch articulation. Sternum unossified. Fore limbs small. 

 Limb and foot bones hollow. Prepubis very short and 

 pointed ; postpubis slender. Femur shorter than tibia. Meta- 

 tarsals elongate. First digit in pes functional ; fifth rudimentary. 



JSTanosaurus. 



Teeth compressed and pointed, and in a single uniform row. 

 Cervical and dorsal vertebrae short and biconcave. Sacral 

 vertebrae three (?). Anterior caudals short. Ilium with very 

 short, pointed front, and narrow posterior end. Fore limbs of 

 moderate size. Limb bones and others very hollow. Femur 

 curved and shorter than tibia. Fibula pointed below. Meta- 

 tarsals very long and slender. 



The genera thus defined contain all the known forms of the 

 the typical Omithopoda from the American Jurassic. They 

 are, moreover, the earliest representatives of this group known 

 in this country from osseous remains, as such fossils have not 

 yet been found in the Triassic, where the oldest Dinosaurs 

 occur. Some of the bird-like footprints in the Connecticut 

 river sandstone may indeed have been made by Dinosaurs of 

 this group, but there is no positive evidence on this point. 

 The American Cretaceous forms of the typical Omithopoda, 

 so far as at present known, are all of large size, and highly 

 specialized, and this appears to be true, also, of the Old World 

 species. 



In considering the relations of this well-marked group, here 

 called the typical Omithopoda, with the other two nearest 

 allied suborders, the quadrupedal Stegosauria and Oeratopsia, 

 it becomes evident, as previously shown by the writer, that all 

 three belong in one great group, which may be regarded as an 

 order. Although differing widely from each other in many 

 notable features, they have a few characters in common, 

 which are important enough to bind them together, and per- 

 haps to indicate a common origin. The most significant of the 

 characters shared by all is the predentary bone, which no other 

 vertebrates possess. Another common character of import- 

 ance, although sometimes nearly obsolete, is a postpubic bone 

 which is present in all Birds, although in some recent forms 

 it is rudimentary. A comparative series showing the relative 

 development of the anterior and posterior branches of the 

 pubis in six genera of American Predentate Dinosaurs is 

 shown in Plate VII. 



