MR. GRAY ON THE STRUCTURE OF CHITONS. 
143 
it assumes the form of an operculum, but is, as in the Chitons, often wanting, espe- 
cially in the adult state. 
Besides having this multiplication of the valves, the parts of the cartilaginous 
mantle not covered by the valves are generally more or less closely covered with 
calcareous scales or subcylindrical spines, which sometimes are so fine as to assume 
the form of more or less flexible bristles or hairs. I cannot call to mind any rnollusca 
which are protected in a similar manner ; the scales and bristles more resemble those 
on the peduncles of certain Cirripedes belonging to the family Pollicipedidce, and 
must be regarded, as the scales are in those genera, as rudimentary valves; their 
form and disposition afford very good zoological characters for the distinction of the 
species. In some, as in the Chiton amiculatus, where the valves are hidden, these 
spines are nearly transparent, and resemble spicula. In others, on each side of the 
mantle, there is a symmetrical series of pores, each armed with a tuft of spicula. 
The spicula may be only modifications of the spines which are found on the surface 
of the mantle in the other species ; but this is a subject worthy of more minute 
inquiry, and I am not aware of any similar spicula being found in any other mollus- 
cous genus, unless they are to be compared to the spicula which are to be found 
imbedded in tlie mantle of Phyllidia, an allied genus ; but the latter are more like 
the spicula of Radiata and Sponges. 
The valves of those kinds (as of Chiton amiculatus of Pallas) which are entirely 
imbedded in the mantle and hidden from external view, are formed much like the 
valves of other rnollusca, of numerous layers of calcareous matter, and they inci ease in 
size by the addition of new material deposited on the inner edge. Their structure is 
compact, hard, heavy and very brittle, and the calcareous matter takes the form of 
crowded perpendicular laminae, placed side by side, diverging from the apex towards 
the edge. As the animal increases in size, these valves are thickened by the addition 
of numerous layers of hard shelly matter, deposited on the inner side, forming a hard, 
glassy concretion. 
But the greater number of species have a part of the valve which is not covered 
by the mantle, but exposed. This exposed part consists of a perfectly distinct ex- 
ternal coat, peculiar I believe to the shells of this family. The outer coat of these 
valves is separated from the lower or normal portion by a small space, filled by a 
cellular calcareous deposit, which is easily seen in a section of the valves, and also 
on the edge of the valves, occupying the space between the inner and outer plate of 
insertion, where it looks like a series of circular holes or tubes ; the space between 
these coats and the cellular internal layer enlarges in thickness and length as the 
valves increase in size and thickness. 
This cellular or tubular structure, as the shell increases in size, also fills up, as far 
as the margin of the outer coat, the notches on the edge of the inner coat which 
separate into lobes the inner part of the valves which are inserted into the cartilagi- 
nous mantle. The filling up of these fissures with this porous matter is peculiar to 
