24 
of tubercles, plates, or spines springing from a disk. The "shagreen" of 
the Dog-fish is composed of closely-set tubercles of dentine-like structure 
with a central cavity. The Thorn-back Kay exemplifies the second variety 
of spines springing from a disk. In some the surface is closely covered with 
these disks ; the spines are traversed by minute tubes like those of dentine. 
All the hard rays in the fins of Acanthopteri belong to the dermoskeleton. 
The curious trigger-like spine in the dorsal fin of JBalistes is an aberrant 
form. The numerous "Ichthyodorulites" which belonged to fossil sharks 
were ancient examples of this kind of weapon. The rostral teeth of the 
saw of the Saw- fish curiously connect the spine of the dermoskeleton with 
true teeth. All teeth however seem properly to belong to the dermoskeleton. 
The brilliant coloring of fishes has been referred to pigment cells containing 
a colored oil. The changing colors of fishes, especially when dying, have 
been celebrated by the poets. 
VIII. 
Analysis of a Report presented by the late Professor Hopkins of 
Cambridge to the British Association "On the Theories of 
Elevation and Earthquakes." 
By William Sanders, F.R.S., F.G.S. 
Read at the General Meeting, November 4ih, 1869. 
In the first part of the paper Mr. Hopkins remarks that a volcano may 
be assumed to consist of a mass of fluid lava, contained in a cavity of the 
solid crust of the earth ; which cavity communicates with the external air 
by means of the volcanic vent. The volcanic mass has an excessively high 
temperature. The formation of elastic gases is constantly taking place— and 
the elastic force generated is extremely great. Respecting the dimensions 
of the cavity, some have supposed that it communicates with a fluid nucleus 
beneath a few leagues of solid crust. To this the author states his objections. 
Others suppose that a communication subsists between volcanoes not far 
removed. But this is refuted by the absence of synchronism in the action 
of neighbouring volcanoes, such as those of Vesuvius, Stroniboli, and Etna. 
There are many proofs of the long continued generation of gases — as at 
Stromboli, where the evolution of vapours has continued for centimes ; also 
at Kirdnei in the Island of Hawayi. The fluidity of lava is supposed to be , 
(not perfect but) impeifect — that a large portion of the lava consists of 
minute solid particles floating in a perfect fluid, which is so sustained by 
