144 
MR. GRAY ON THE STRUCTURE OF CHITONS. 
this family ; for the fissures on the margin of the shells of the genus Emargmula, 
of Pleurotoma, of Pleurotomaria, and of the holes formed by the notches in the 
edge of the lip of Haliotis, are filled up by layers of shelly matter of the same 
structure as the rest of the shell, and deposited in the same manner. These fissures, 
filled with this porous substance, may be seen on the inner surface of the valves 
forming lines of pores diverging from the tip, and increasing in width as they ap- 
proach the edge, to the notches above referred to : they are useful in a zoological 
point of view, as showing the number of lobes into which the inner plate of insertion 
is divided. 
The valves of the species whose shells are covered by the mantle have a plain edge, 
with only one or two notches. 
Those of which the shells are partly external have the inner coat of the valve pro- 
duced beyond the outer coat, thus forming what is called the plate of insertion ; for the 
valves of these animals do not simply cover the mantle as with a case, but have their 
edge inserted into the cartilaginous mantle, another character peculiar to this group. 
The inner plate of insertion, besides being divided into lobes by the fissures or 
slits above mentioned, has the edges of the lobes divided into deep grooves or pecti- 
nations. 
The edge of the outer coat, which is never slit or lobed, is similarly but not so 
strongly pectinately divided, where it is inserted into the mantle. 
This kind of edge is probably produced by the perpendicular radiating laminae of 
which the two coats are formed, the number of teeth appearing to agree with the 
number and thickness of the plates, the teeth and laminae being thicker and more 
numerous on the outer than on the inner coat. 
This kind of edge does not occur, as far as my observation extends, in any other 
mollusca, for it is very unlike the grooves on the edge of many bivalves which are 
formed by the processes on the surface of the mantle, and more resembles the plates 
between the tubes in the substance of the valves of some Barnacles (Balani), but 
has no real resemblance to them in structure. 
The more prominent peculiarities of this family appear to be, — 
1 . That instead of having a single valve, as is the case with most Gasteropodes, 
they have a series of more or less perfect valves placed in front of the normal valve, 
the front one being the most imperfect, all imbricated the one over the other. 
2. Besides this increase in the number of valves, the surface of the mantle is 
covered with numerous rudimentary valves assuming the form of scales or spines. 
3. These spines are sometimes placed in tufts symmetrically dispersed on the sides 
of the body. 
4. The valves of the more normal Chitons, which are partly exposed, are furnished 
with two additional coats, of the size of the exposed part, not found in the shells of 
any other mollusca, the intermediate coat being of a porous texture ; and this coat 
fills up the symmetrical slits usually found in the innermost coat. 
