82 0. " Marsh—Classification of the Dinosauria. 
regarded, not as an order, but as a sub-class, and this rank is 
given it in the present communication. The great number of 
subordinate divisions in the group, and the remarkable divers- 
ity among those already discovered indicate that many new 
forms will yet be found. Even among those now known, there 
is a much greater difference in size and in osseous structure 
than in any other sub-class of vertebrates, with the single 
exception of the placental Mammals. Compare with the 
eel land animals kno one or sixty feet long, down to 
some of the smallest, a fae inches only in length. 
According to present evidence, the Dinosaurs were confined 
entirely to the Mesozoic age. They were abundant in the Tri- 
assic, culminated in the Jurassic, iad continued in diminishing 
numbers to the end of the Cretaceous period, when they became 
extinct. The great variety of forms that flourished in the 
ioral render it more than probable that some members of 
e group existed in the Permian period, and their remains 
may be brought to light at any time. 
riassic Dinosaurs, although so very numerous, are 
‘cae to-day mainly from fonbptiatn and fragmentary osseous 
remains. Not more than half a dozen skeletons, at all com- 
plete, 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- 
Sher y- he main difficul ty at present with the Jurassic 
nities of the diminutive 
rina which appear to approach Birds so amg These 
orms we oO thei ] 
mostly fragmentary, and can with difficulty “ai aiden presale 
from those of Birds, which occur in the same beds. Future 
discoveries will, without doubt, throw much light upon this 
oin 
Comparatively little is yet known of Cretaceous Dinosaurs, 
a many have been described from incomplete speci- 
speciali zed. 
