370 John Young—NStructure of Shell of Chonetes. 
tions are large and wide set, being most numerous around the outer 
margin of the valves, and are placed 1 in single rows between each of 
the ribs, whereas the tubular openings, or bases of the spines, seen on 
the outer surface of the shell, are all planted on the ribs. 
4- 
. Inner perforations between the ribs. 
. Outer surface of shell when slightly etched. 
. Vertical section showing the minute perforations and spine-pores. 
. Vertical section showing the inner perforations between the ribs. 
. Inner surface of shell, etched to show the perforations. 
All the sketches more or less magnified. 
nH co bo re 
There can, I think, be little doubt about these inner perforations 
being tubular, as in some specimens they are found to be filled with 
a darker mineral matter than the substance of the shell, and in thin 
sections the shell-structure is seen to be arranged around the open- 
ings in minute waving or concentric lines of growth. This inner 
layer of shell, around the perforations, is dense in its structure, and 
is penetrated with no smaller series of pores than those seen between 
the ribs. In this it agrees in its inner shell-structure with several 
of the species of Productus, in which, also, as I have already shown, 
the inner series of perforations never did reach the outer shell-surface.' 
The row of tubular spines along the cardinal edge of the ventral 
valve of this species of Chonetes reveals some interesting evidence as 
to their arrangement, which I do not think has been before recorded. 
These spines pass right through the thickness of the shell, as tubes, 
and open on the inner surface, the tubes being often filled with 
foreign mineral matter, which, being generally darker in colour than 
the substance of the shell, reveals the path of the tubes through the 
shell in the clearest manner. It is well known that the series of 
cardinal spines in Chonetes diverge from the beak on either side the 
valve, in a slanting manner, towards the outer extremity of the 
shell; but I do not think it is known that in their inner path 
through the shell, that the tubes slanted in the opposite direction, so 
as to point, or converge towards the beak. 
Specimen showing nine spines on either side the beak, enlarged one diameter. 
This is beautifully shown in some of the etched specimens that 
I have prepared. In these it is seen that the spines nearest 
1 Davidson’s Brachiopoda, part iv. pp. 296-301. 
