56 
THE SILURIAN BEACH. 
There is another type of these ancient Verte- 
brates which has also its representatives among 
our modern fishes. These are the Sharks and 
Skates, or, as the Greeks used to call them, the 
Selachians, — making a very appropriate distinc- 
tion between them and common fishes, on ac- 
count of the difference in the structure of the 
skeleton. In Selachians the quality of the bones 
is granular, instead of fibrous, as in fishes ; the 
arches above and below the backbone are formed 
by flat plates, instead of the spines so character- 
istic of all the fish proper ; and the skull consists 
of a solid box, instead of being built of overlap- 
ping pieces like the true fisli-skull. They differ 
also in their teeth, which, instead of being im- 
planted in the bone by a root, as in fishes, are 
loosely set in the gum without any connection 
with the bone, and are movable, being arranged 
in several rows one behind another, the back rows 
moving forward to take the place of the front 
ones when the latter are worn off. They are un- 
like the common fishes also in having the back- 
bone continued to the very end of the tail, which 
is cut in uneven lobes, the upper lobe being the 
longer of the two, while the terminal fm, so con- 
stant a feature in fishes, is wanting. The Se- 
lachians resemble higher Vertebrate types not 
only in the small number of their eggs, and in 
the closer connection of the young with the 
