. 
244 
NATURE 
rg 
[ Fan. Xo, 1882 
I form a less hopeful prognostication. I think the 
ycean basins are the scar, which still testify to the place 
of separation. 
The density of the moon is 0°56654 times that of the 
earth. Putting the mean density of the earth at 5°5, this 
makes the density of the moon 31. The density of 
granite is about 2°68, and that of basalt 2°96. Conse- 
quently the density of the moon is a little greater than 
that of the basic layer of the earth’s surface, which I 
think we may expect to occur at the sea-board at a depth 
of about 25 miles. The entire mass of the moon is 
0'011364 of the mass of the earth. 
Accordingly, it would require a layer of about 31 
miles thick, of the density of granite, to be taken off the 
surface of the primitive mass to make a body of the mass 
of the moon; and if the mean density of the matter re- 
moved was the same as that of the moon, a somewhat 
thinner layer would suffice. But if we reduce the area of 
the skin removed to the area of the oceans, it would 
197 
146 
uniform layer rather less than 41 miles thick taken off the 
oceanic areas would be sufficient to make the moon. 
Of course the layer removed would not, in fact, have 
been of uniform thickness. But the above estimate 
gives an idea of the size of the cavity which would be 
produced. What then would happen? This would de- 
pend upon whether the surface had already become at all 
solid. I conceive this would be the case at a very early 
stage, judging from the manner in which a solid layer 
forms on the liquid lava of Kilauea. The hole would 
therefore fill up by the rise of the liquid from below, 
rather than by the lateral approach of the edges of the 
wound.- When the raw surface again solidified we should 
have a crust of greater density over the area in question, 
because formed from a lower and denser layer, which 
would have risen not quite to the level of the lighter 
crust. There would, however, have necessarily been a 
certain amount of flow in the upper fluid layers towards 
the cavity, and this would have carried the cooled granitic 
crust which, floating on it, still remained upon the earth 
along with it. What was left of the granitic crust would 
therefore be broken up into fragmentary areas, now re- 
presented by the continents. This would make the 
Atlantic a great rent, and explain the rude parallelism 
which exists between the contours of America and the 
Old World. 
The sudden rupture of so considerable a fragment from 
the rotating spheroid, would alter its mass, form, and 
moment of momentum. It appears then that its axis of 
rotation would be altered, which might account for the 
require to be X 31, or about 41 miles deep. Hencea 
fact, that the approximate pole of the oceanic area is | 
not in the equator. 
The volcanic surface of the moon, if volcanic it be, 
would lend considerable support to the view which I 
maintain, that the water substance emitted by volcanoes 
is an integral constituent of the fluid substratum. For 
when the moon broke away from the earth it would 
carry with it the aqueous constituent of the magma. 
| 
Owing to the much smaller force of gravity in the moon, | 
the pressure under which this would there be placed 
would be much less than in the earth. Consequently it 
would more easily escape, and the signs of volcanic 
action would be more pronounced. But the difficulties 
surrounding terrestrial vulcanism are so great, that one is 
hardly tempted to add the lunar to them. 0. FISHER 
CLASSIFICATION OF THE DINOSAURIA* 
_ 
Science (p. 423) 1 presented an outline of a classifica- 
tion of the Jurassic Dinosaurian reptiles of this country | 
1 By Prof. O. C. Marsh. Read before the National Academy of Sciences, 
at the Philadelphia meeting, November 14, 1882. Communicated by the 
Author. 
the May number of the American Journal of 
which I had personally examined. The series then 
investigated is deposited in the Museum of Yale College, — 
and consists of several hundred individuals, many of 
them well preserved, and representing numerous genera 
and species. To ascertain how far the classification pro- 
posed would apply to the material gathered from wider 
fields, I have since examined various Dinosaurian re- 
mains from other formations of this country, and likewise 
during the past summer have visited most of the museums — 
of Europe that contain important specimens of this 
group. Although the investigation is not yet completed, 
I have thought the results already attained of sufficient 
interest to present to the Academy at this time. 
In previous classifications, which were based upon very 
limited material co.npared with what is now available, 
the Dinosaurs were very generally regarded as an order. 
Various characters were assigned to the group by von 
Meyer, who applied to it the term Pachypoda ; by Owen, 
who subsequently gave the name Dinosauria, now in 
general use; and also by Huxley, who more recently pro- 
posed the name Ornithoscelida, and who first appreciated 
the great importance of the group, and the close relation 
it bears to birds. The researches of Leidy and Cope in — 
this country, and Hulke, Seeley, and others in Europe, 
have likewise added much to our knowledge of the 
subject. 
An examination of any considerable portion of the 
Dinosaurian remains now known will make it evident to 
any one familiar with reptiles, recent or extinct, that this 
group should be regarded not as an order but as a sub- — 
class, and this rank is given it in the present communica- — 
tion. The great number of subordinate divisions in the — 
group, and the remarkable diversity among those already a 
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 excep- _ 
tion of the placental Mammals. Compared with the 
Marsupials, living and extinct, the Dinosauria show an 
equal diversity of structure, and variations in size from by 
far the largest land animals known—fifty or sixty feet 
long, down to some of the smallest, a few inches only in 
length. : 
According to present evidence the Dinosaurs were con- 
fined entirely to the Mesozoic age. They were abundant 
in the Triassic, culminated in the Jurassic, and continued 
in diminishing 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 be brought to 
light at any time. 
The Triassic Dinosaurs, although so very numerous, 
are known to-day mainly from footprints and fragmen- 
tary osseous remains. Not more than _half-a-dozen 
skeletons, at all complete, have been secured rrom de- 
posits of this period; hence, many of the remains 
described cannot at present be referred to their appro- 
priate divisions in the group. 
From the Jurassic period, however, during which Dino- 
saurian reptiles reached their zenith in size and numbers, 
representatives 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 the entire 
osseous structure of typical examples, at least, be made 
out with certainty. The main difficulty at present with 
the Jurassic Dinosaurs is in ascertaining the affinities of 
the diminutive forms which appear to approach birds so 
closely. These 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 Dino- 
