80 Prof. 0. C. Marsh on the Dinosaur ia. 



far the largest land animals known (50 or 60 feet long) clown 

 to some of the smallest, a few inches only in length. 



According to present evidence, the Dinosaurs were confined 

 entirely to the Mesozoic age. They were abundant in the 

 Triassic, culminated in the Jurassic, and continued in dimi- 

 nished numbers to the end of the Cretaceous period, when 

 they became extinct. The great variety of forms that 

 flourished in the Triassic render it more than probable that 

 some members of the group existed in the Permian period ; 

 and their remains may at any time be brought to light. 



The Triassic Dinosaurs, although so very numerous, are 

 known today mainly from footprints and fragmentary osse- 

 ous remains. Not more than half a dozen skeletons at all 

 complete have been secured from deposits of this period ; 

 hence many of the remains described cannot at present be 

 referred to their appropriate divisions in the group. 



From the Jurassic period, however, during which Dino- 

 saurian reptiles reached their zenith in size and numbers, re- 

 presentatives of no less than four well-marked orders are 

 now so well known that different families and genera can be 

 very accurately determined, and almost tlie entire osseous 

 structure of typical examples, at least, be made out with 

 certainty. The main ditheulty at present with the Jurassic 

 Dinosaurs is in ascertaining the affinities of the diminutive 

 forms which appear to aj^proach birds so closely. Tiiese 

 forms were not rare ; but their remains hitherto found are 

 mostly fragmentary, and can with difficulty be distinguished 

 from those of birds, which occur in the same beds. Future 

 discoveries will, without doubt, throw much light upon this 

 point. 



Comparatively little is yet known of Cretaceous Dinosaurs, 

 although many have been described from incomplete speci- 

 mens. All of these appear to liave been of large size, but 

 much inferior in this respect to the gigantic forms of tiie 

 previous period. The remains best preserved show that, 

 before extinction, some members of the group became quite 

 highly specialized. 



ilegarding the Dinosaurs as a subclass of the Reptilia, 

 the forms best known at present may be classified as fol- 

 lows : — 



Subclass DINOSAURIA. 



Premaxillary bones separate ; upper and lower temporal 

 arches; rami of lower jaw united in front by cartilage only; 

 no teeth on palate. Neural arches of vertebrae united to centra 

 by suture ; cervical vertebras numerous ; sacral vertebrae co- 



