In the paper published in the last No. of the Journal, I gave a list of 60 species of Diatoms observed in the Mull deposit, or rather in one portion of it, by the Rev. W. Smith, adding that I had myself, in other portions, seen not only all these, but others which Mr. Smith did not meet with. Since then I have examined with care several different portions of the deposit, which, as I mentioned, varies a good deal in the relative proportion of species in different parts, and have detected upwards of 30 additional forms, most of which I have been enabled easily to identify by the aid of Mr. Smith’s Synopsis. In mentioning my observations, I shall use the names given in that work; not, however, meaning by this to assert, that all Mr. Smith’s species are true and permanent species, for this is not the opinion of that author himself; but only with a view to the subsequent identification of the observed forms with those so beautifully figured by Mr. West, and their comparison with forms from other localities.
But before proceeding I have to apologize for, and to correct, several errata in the former paper, chiefly in the list of species. My absence on the continent prevented my seeing a proof; and for the same reason, the figures of the new form were not seen by me after they were engraved, and owing to their having been taken from hurried sketches never meant for publication, they are not such as I could have desired.
Perhaps the best plan will be to repeat here the list of forms given in the former paper, and then to subjoin my new observations. These forms were—
With respect to the new form previously described, Mr. Smith has proposed, since my former paper was written, to call it, provisionally, Eunotia incisa (No. 61), the notches which it exhibits forming a very well marked character. The figures formerly given of the two modifications or varieties of this form not being satisfactory, I here give such as will better indicate their true character.‡ My reasons for including the two forms together are, first, that both invariably exhibit the notches; secondly, that the number of striæ appears to be the same in both, while the general aspect undoubtedly is so; and thirdly, that many specimens occur in which one apex is narrow, as in fig. 1, while the other is rounded, as in fig. 2. I would add a remark with respect to a point which appears characteristic; that while the form No. 1 is quite symmetrical,No. 2 very rarely, if ever, is so, one end being always broader than the other. From the greater width of No. 2, the striæ are much more easily seen in that form than in No. 1.
Since my paper was written, 1 have detected this form in one more deposit, besides the present one and that mentioned in my former paper as said to be from the banks of the Spey; namely, in the Bergmehl of Lillhaggsjön in Lapland. It is not so abundant there as in the Mull deposit; and while, in the latter, No. 1 is the more frequent form, in the former, the Lapland earth, No. 2, is more common.
I shall now give a list of those additional forms observed by me, in a careful study of several different portions of the deposit, which can be readily referred to figures in Mr. Smith’s Synopsis. These are—
Of the above 27 forms, the only ones which do not entirely agree with Mr. Smith’s figures are those I have termed Navicula obtusa and Epithemia Sorex. The latter is nearer in form to E. Musculus, but as that is a marine form, I have preferred the other name, provisionally. In my specimens this form is very scarce, but may be found more abundantly in others. As to N. obtusa, the form so named by me, of which I give figs., Nos. 3 and 4, is rather more like N. affais, but is one half larger than either of the two as figured in the Synopsis. As, however, N. obtusa is said to occur in the Lough Mourne deposit, and as I find there the form here figured, precisely as in the Mull earth, and no other resembling it, I have chosen the name N. obtusa in the mean time. It is clearly distinct from all the Naviculæ figured in the Synopsis, except perhaps the two just named.
We have thus 88 distinct Diatomaceous forms (whether in all cases true species or not, is a matter for future decision) in this remarkable deposit. The study of it has led me to think it probable that several forms, at present separated, will have to be united; but our knowledge of Diatomaceous forms and of their modifications is not yet sufficient to enable us with certainty to classify them all. There is nothing, therefore, to be done, but to describe and accurately to figure, all such forms as appear distinct, and we shall thus in time be able to trace out the relations among them. This very deposit appears to me rich, not only in distinct forms, but in modifications of these, in several cases exactly intermediate between, the figures of recorded species. To this part of the subject I shall return at a future time; for the present, I confine myself to mentioning the forms which agree with the published figures.
It will be observed, that 27 species and 2 genera, Cocconema and Cyclotella, have already been added to the former list. But I am quite sure that the number is not yet exhausted, for I have observed several well marked forms, which I cannot securely refer to any of those figured in the Synopsis, and which may probably, therefore, prove to be new to Britain. Among these are one or two Pinnulariæ; one or two Naviculæ; one, perhaps two, Sgnedræ; one Nitzschia, possibly two; one which I take to be a Melosira, but for want of the 2nd volume of the Synopsis I cannot compare it with figures of the British species. The same remark applies to the genus Himantidium, of which Mr. Smith identified 6 well marked species, but of which, or of some allied forms, I have reason to think two, perhaps three more are present. I shall hereafter describe and figure all such doubtful forms.
For the present, I shall conclude with the description of a very distinct and well marked species of Pinnularia, on the specific character of which no doubts can be entertained, and which is therefore new to Britain, if not to science. This form I early noticed, but it was not till I had compared it with the figures in the Synopsis that I felt sure of its being different from all the forms in that work, as well as from all figures of Pinnulariæ known to me. In the Mull deposit, it occurs in all the different portions I have yet examined, but invariably very widely scattered, so that a good slide, rich in forms, seldom yields more than one or two individuals, and occasionally contains none at all. Hence, and from its small size, it is apt to be overlooked, except in a very minute and careful search. It is of course impossible for me to say whether this form have been already described as occurring in foreign countries, but as yet I have seen neither figure nor description to which it can be referred. I would propose, therefore, pro visionally, to name it Pinnularia hebridensis; and if any of your readers should recognise it as one already named, of course the earlier name must be adopted.
Pinnularia hebridensis.—V. elliptical, narrow, almost rectangular, with rounded ends, sometimes very slightly constricted in the middle, and sometimes very slightly acuminate at the apices. F. V. rectangular, with the corners slightly rounded. Length, from · 00125 to · 0025. Costæ strong, distant, radiated at the middle, not nearly reaching central line, 10 to 11 in-001. Habit stout, notwithstanding its narrowness, so that it seldom occurs fractured. The figures 5 and 6 will give some idea of its aspect.
The small size, for this is one of the smallest Pinnulariœ, combined with the strong distant costæ, at once distinguish it from all those figured by Mr. Smith. I find in the late edition of Pritchard’s ‘Infusoria,’ a description of Stauroptera scalaris, Ehr., which has some points of agreement with the above, such as the small size and the distant costa. But the figure of the valve (Prichard, pl. xv., fig. 10) is very much broader, and the number of costæ is said by Ehrenberg to be 12 in 1-1200, which is = 14 in ·001, whereas my form has usually 10 only, sometimes only 9 · 5. Besides this it has not the pseudo-stauros which marks the genus Stauroptera of Ehrenberg, and the nature and form of the nodules and median line correspond exactly to those of Pinnularia alpina, while the arrangement of the costæ is also very similar to what is seen in that species, only on a very small scale; the form, however, is quite different.
I have only to add that, hitherto, I have been unable to detect the presence of this form in any other deposit which I have had an opportunity of examining; and that if any of your readers can throw light on the subject, or has observed any other well marked species in the Mull deposit, I shall feel deeply indebted to them if they will make known their observations. I shall also be happy to supply observers with the material for their researches.
I have added this variety as it was noticed in the former paper.
This species was accidentally omitted from the former list.
These figures will be given in the next number of the Microscopical Journal.