It was a matter of considerable interest to determine the extent of country to which these corrections would apply, and to this purpose I have had recourse to the observations made at different parts of the country and furnished to the RegistrarGeneral. Some of the results of this investigation I have already mentioned in the remarks following Table III. The general result was found to be, that for all places situated inland, the values contained in these tables may be adopted at once. For places situated near the sea the hygrometrical values may not be strictly true, but in the absence of any series of observations taken in these localities from which the corrections can be deduced, we must arrive at approximate mean values by means of the observations at the Royal Observatory at Greenwich. In the reduction of the observations for the Registrar-General I have so done; and I have found the tables to be of great assistance, not only in the reduction of the observations, but also in the detection of errors, and pointing out the place where such existed.

I have merely to remark that I have not formed tables of corrections for longer periods than a month, as the doing so would have extended this paper to a great length, and such can be readily formed from the tables themselves; neither have I spoken of the mean or other values, as I hope soon to have the honour of presenting to this Society some of the meteorological results deduced from the observations taken at the Royal Observatory between the years 1840 and 1845.

Greenwich, Feb. 10, 1848.

IX. On the Structure of Chitons. By J. E. GRAY, Esq., F.R.S., F.Z.S., &c.

Received June 17,-Read June 17, 1847.

LINNÆUS and most of his successors arranged the Chitons with the Pholades and the Balani, as multivalve shells. ADANSON, with his usual tact, placed them with the Patella, and the anatomy of the animal, published by POLI and CUVIER, has shown the propriety of this position. M. DE BLAINVILLE separated the Chitons from the other Mollusca, with which they had always been placed on account of their possessing a series of imbricated shelly valves arranged along the central line of their back, and placed them with the Cirripedes in a peculiar subclass, which he called Articulated Mollusca, and considered as intermediate between the two divisions of the animal kingdom. I need scarcely observe that this division has not been adopted, the Cirripedes having now been proved to be true Crustacea.

There are a few peculiarities in the internal structure of these animals not found in other Mollusca. The shells also offer some differences, which have not, as far as I am aware, been previously noticed, and which it is the object of this paper to describe.

The Chitons at first sight differ from all the other Mollusca in being provided with a series of imbricated valves, forming a line down the centre of the back, and in this character they differ essentially from the Patella, to which they are most nearly allied in the general form, the position of the gills, &c. I have therefore thought it desirable to examine these valves with reference to one another, and to determine which most resemble the valves usually found on other Mollusca, and which are the additional or supplementary valves.

The posterior terminal valve (fig. 1, C and F, fig. 2, C) of the more normal Chitons, as in the restricted genus Chiton for example, most nearly resembles the conical form which the valves of Mollusca generally assume, and the other valves are only modifications of the same form.

The seven anterior valves are formed like the posterior one, but with the greater part of the hinder half deficient, and with the front edge of insertion somewhat enlarged. In the front valve (figs. 1 and 2, A and D) the anterior

F

Fig. 1.

Chiton striatus.

Fig. 2.

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 insertion, 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 interrupted, 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, k); the raised edges (figs. 1 and 2, ) on the medial valves which separate the front of the valves into what has been Enoplochiton niger. 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 posterior 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 Chitonida is exactly what we might have expected à 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

it assumes the form of an operculum, but is, as in the Chitons, often wanting, especially 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 mollusca which are protected in a similar manner; the scales and bristles more resemble those. on the peduncles of certain Cirripedes belonging to the family Pollicipedidae, 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 molluscous genus, unless they are to be compared to the spicula which are to be found imbedded in the 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 mollusca, of numerous layers of calcareous matter, and they increase 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 laminæ, 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 external 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 cartilaginous mantle. The filling up of these fissures with this porous matter is peculiar to