Prof. O. C. Marsh—On Stegosaurus. I 
researches serves to convince us, however, that such rise of 
temperature is by no means a necessary prelude to a volcanic out- 
burst, but that the percolation of water to a centre of igneous 
activity is in itself sufficient to serve as an exciting cause. If we were 
to imagine a mass of solid nitre lying at some depth within the 
earth’s crust, and having a temperature of 260° C., such a mass 
would be solid and inert. Butif, without raising the temperature, a 
quantity of water, equal to five per cent. of the weight of the nitre, 
were introduced into it, then the whole would become liquid and 
the tendency of the heated water to relieve itself from the pressure 
might give rise to all the phenomena of a volcanic outburst. What is 
true of nitre is equally true of the mixed silicates composing lavas, 
which are at much higher temperatures. ‘The admission of water to 
such a mass of mixed silicates at a temperature below its point of fusion 
would cause it to become liquid and thus give rise to the phenomena 
of eruption. In the case of the Krakatoa-lavas, the anhydrous 
varieties probably existed quite solid at tolerably high temperatures ; 
but the gradual percolation of water into their mass (and the 
evidences of such percolation, are seen in the hydrated condition of 
the minerals composing it) would render the whole liquid, without 
the necessity for any rise in temperature. 
I1.—Tue Sxuut anp Dermat Armour oF STzZGOSAURUS. 
By Professor O. C. Marsu, Ph.D., LL.D. 
(PLATES I. II. III.) 
N various numbers of the “American Journal of Science,” the 
writer has given the more important characters of the skeleton 
of the Steyosauria, and has indicated the relations of this group to 
the other known Dinosauria.!. The discovery of additional specimens 
of Stegosaurus, one of them nearly complete, furnishes material to 
greatly enlarge our knowledge of the skull and dermal covering of 
this genus, and some of the new facts are given in the present 
article. 
The results of the entire investigation of this group will be 
brought together in a monograph now in preparation, by the writer, 
for the United States Geological Survey. The lithographic plates for 
this volume, sixty-five in number, are nearly all printed, and the 
figures of the skull here given are taken from these plates. 
Tue Sxuuz. (Plate 1.) 
The skull of Stegosaurus is long and slender, the facial portion 
being especially produced. Seen from the side, with the lower jaw 
in position, it is wedge-shaped, with the point formed by the pre- 
maxillary, which projects well beyond the mandible, as shown in 
Fig. 1, Plate I. The anterior nares (a) are large, and situated far 
in front. The orbit (6) is very large, and placed well back. The 
1 « American Journal of Science,’’ vol. xiv. p. 513, Dec. 1887; vol: xix. p. 253, 
March, 1850; vol. xxi. p. 167, Feb. 1881; vol. xxiii. p. 83, Jan. 1882; and 
vol. xxiv. p. 410, Noy. 1887. 
