142 
MR. GRAY ON THE STRUCTURE OF CHITONS. 
wings of insertion found fully developed in the intermediate valves (figs. 1 and 2, B 
and E) are reduced, and the teeth (figs. 1 and 2, g), which are 
generally to be seen in the middle portion of the front edge of 
the posterior and intermediate valves between the wings of inser- 
tion, are greatly enlarged, forming an uniform edge extending 
along the entire outer and anterior boundary of the valve. 
The lobes on the margin of the front valve are generally to be 
seen distinctly developed, but of a smaller size on the front edge 
of the central portion of the posterior and medial valves (figs. 1 
and 2, g ) ; and they are generally most developed on the valve 
next to the front one ; from the notches (figs. 1 and 2, h) between 
these lobes in the edge of the front valve, there issue series of 
pores; and similar series of pores, but more crowded and inter- 
rupted, are to be observed in the middle of the inner surface of 
the intermediate valves (figs. 1 and 2, g), showing the truth of this 
comparison. The notch (figs. 1 and 2, i) on the hinder part of 
each wing of insertion of the intermediate valves is evidently the 
same as the front notch on the edge of insertion of the hinder 
valve (fig. 1, A-) ; the raised edges (figs. 1 and 2,1) on the medial 
valves which separate the front of the valves into what has been 
called the lateral and central area, are analogous to the more or less distinct keel 
(figs. 1 and 2, m) which passes from the apex to the front edge of the margin of the 
hinder valve. 
* 
In the more abnormal Chitons, as in the genus Katharina, which have the apex of 
the posterior valves lower down, and produced beyond the margin, arising from the 
hinder portion of the posterior valve being more or less abortive, while the front 
portion is more than usually developed, the hinder valve loses its peculiar character 
and more nearly resembles the usual form of the intermediate valves, and the pos- 
terior edge of the hinder valve has only a small simply striated or smooth edge of 
insertion in the place of the lobed plates of insertion of the other genera. 
The result of this examination of the structure of the valves of the Chitonidce is 
exactly what we might have expected a priori, though it has not, that I am aware, 
been observed by any preceding author, and did not occur to me until after the 
comparison of the valves with one another. That is to say, the posterior valve 
which is placed over the more important organs is generally the most fully developed, 
and is the homologue of the shell of the Patella, while the others which are arranged 
in front of it are more imperfect, and the front one is the most rudimentary of the 
series. The Chitons may therefore be considered as normal Gasteropodes, with a 
series of more or less rudimentary valves in front of the usual shell. These additional 
valves appear to have no relation to the second lateral valve found in the normal or 
bivalve Mollusca, or to its representative in the Gasteropodes, where in some genera 
Enoplochiton niger. 
