marsh.] HALLOPUS. 153 



appearance; while others still, diminutive in size and of light and grace- 

 ful form, were so much like birds that only a comparative anatomist, 

 with well-preserved skeletons of both before him, could tell one from 

 the other. Iu this case, at least, a single tooth or bone would not 

 suffice, though a Cuvier sat in judgment. 



In the western part of this country, especially in the Rocky Mountain 

 region, vast numbers of dinosaurs lived and nourished during all Juras- 

 sic time. Their remains are so abundant, and so perfectly preserved 

 in many localities, that those already obtained have furnished the basis 

 for a classification of the whole group. This classification, first pro- 

 posed by the writer in the American Journal of Science in 1881, and 

 subsequently emended, ruay be appropriately used here iu considering 

 the American dinosaurs from this formation. It will be discussed more 

 fully in the concluding part of the present paper. 



THEROPODA. 



Xear the base of the Jurassic in the Rocky Mountain region an 

 interesting geological horizon has been defined as the Hallopus beds, 

 since here only remains of a remarkable dinosaurian, named by the 

 writer Hallopus victor, have been found. The position of this horizon 

 is shown in the diagram on page lio (fig. 1). Another reptile, Xano- 

 saurus, the most diminutive dinosaur known, occurs in the same strata. 

 This horizon is believed to be lower than that of the Baptanodon beds, 

 although the two have not been found together. The Hallopus beds 

 now recognized are in Colorado, below the Atlantosaurus beds, but are 

 apparently quite distinct from them. 



HALLOPUS. 



The type specimen of Hallopus, the only one known, is the greater 

 part of the skeleton of an animal about the size of a rabbit. This was 

 described by the writer in 1877, and referred to the Dinosauria. On 

 further investigation it was found to be distinct from all the known 

 members of that group, and in 1881 it was made the type of a new 

 suborder, the Hallopoda. One of the most distinctive characters, 

 which separated it widely from all known dinosaurs, was seen in the 

 tarsus, which had the calcaneum much produced backward. This 

 feature, in connection with the greatly elongated metatarsals, suggested 

 the generic name Hallopus, or leaping foot. 



The general structure of the pelvis, especially of the ilium and pubis, 

 as well as the proportions of the entire hind limb, suggested an affinity 

 with Compsognathus, from the Jurassic of Bavaria, and the writer, in 

 his classification of the dinosaurs, in 1882, placed the Hallopoda next 

 to the suborder Compsognatha, which belongs in the great group of 

 carnivorous dinosaurs, the Theropoda. 



The writer has since reexamined the type specimen and had various 

 parts of it uncovered, so far as the hard matrix of red sandstone would 

 permit. This has brought to light other portions of the skeleton, so 



