John Young—Structure of Shell of Chonetes. 369 
of Hast Kilbride, Lanarkshire. The fossils in this shale have their 
structure generally well preserved, there being less change, through 
crystallization of the lime present in the shells of the various 
organisms, than is found in those obtained from most of the other 
fossiliferous localities in Western Scotland. 
When we examine the outer surface of the shell of this Chonetes 
in well-preserved specimens, we find that the numerous ribs are 
comparatively smooth, and that in most specimens they are crossed 
by a few concentric lines of growth, which are generally placed 
near the outer margin of the valves. The ribs are also per- 
forated to a certain depth, and at irregular intervals, by a slanting 
series of tubular openings, that were probably occupied by the bases 
of small spines. They are variable in number, on the average from 
8 to 10 in the length of each rib. If the outer surface of such a speci- 
men is shghtly etched with weak acid, so as to remove a thin film of 
the shell, we then see that each rib is ornamented with a single row 
of very minute and closely-set tubercule-like pores, that descend 
vertically for a short distance into the substance of the shell, and 
then disappear as the surface is etched a little deeper. Whether 
these minute perforations ever opened at the surface of the shell as 
pores, it would now be very difficult to say, as the openings appear 
to have been too small, to admit any foreign mineral matter into the 
substance of the shell, by which such external openings could be 
readily recognized from the ordinary calcite of the shell. 
When a specimen is examined with a pocket-lens, and is held so 
that the light shall pass through the ribs cross-wise in the direction 
of their length, the perforations are then seen to extend downwards 
into the substance of the shell through a layer representing about 
the thickness of the ribs. When looked at in a vertical direction under 
a low power of the microscope, they appear on the surface of the ribs 
as minute rows of tubercules slightly raised above the level of 
the shell. These tubercules are so close to one another, that their 
bases almost touch each other, and in the layer of the shell in which 
they have been formed, it is evident that the shell-structure, during 
its growth, has been modified to a certain extent, so as to allow of 
its arrangement around the perforations into the form of minute 
tubercules. ‘This beautiful and interesting structure I have only 
found after etching some of my finest preserved specimens: it seems 
to be destroyed in those specimens where the shell is much crystal- 
lized. 
In the larger, slanting, tubular openings found on the ribs, these, 
in etched specimens, are seen to be filled with mineral matter different 
from the calcite of the shell, and stand out on the surface as small, 
short spines. They do not pass through the thickness of the shell, 
but disappear, as the surface of either valve is etched, to near the 
middle layer. At this point a new series of perforations are seen in 
the shell-structure which do not reach the outer surface, but pass 
inwards, in an upward slanting direction, and open on the interior of 
the valves as numerous, raised, blunt tubercules, that give the inner 
surface quite a roughened appearance. ‘This inner series of perfora- 
DECADE II.—VOL. X.—NO. VIII. 24 
