On some New and Rare Diatomaceæ from the West Coast of Ireland

Some of the common marine species are met with ; for instance—

It is a remarkable fact that the above-named species are relatively few, and the forms belonging to them, generally speaking, are not of frequent occurrence.

Besides the common forms just enumerated, I have found a large number of the rarer species described by Donkin, Gregory, Greville, and Roper, investigators in this department of natural science whose discoveries have been made known since the publication of Smyth’s ¹ Synopsis of British Diotamaceæ,’ namely—

As regards the forms included in the foregoing list, I have no remark to make beyond the record of their occurrence, except in the case of Campylodiscus simulans and Coscinodiscus nitidus. Several frustules of Campylodiscus simulans have occurred in the gathering, and in many instances I have observed the same peculiarity which Dr. Gregory noticed in the frustules of Campylodiscus bicruciatus, namely, that the opposite valves are frequently placed at right angles to each other.

Coscinodiscus nitidus is figured and describedby Dr. Gregory in his paper on “New Forms of Diatomaceæ found in the Frith of Clyde,” and supposed by him to be the same as a form previously figured from an imperfect specimen found in the Glenshira Sand. In the paper on the Clyde forms Dr. Gregory, having described Coscinodiscus nitidus, proceeds to say, “This pretty disc was figured without a name from an imperfect specimen in my last paper on the Glenshira Sand. Trans. Mic.Soc.’ Vol.V, P1. I, fig.50.) Havingfound it tolerably frequent in Lamlash Bay, I now figure a perfect example, which, provisionally, I refer to Coscinodiscus.”

This form found in Lamlash Bay occurs frequently in Dr. Wright’s gathering, and with equal frequency is another form very Eke it at first inspection, but which, on closer examination, presents distinctive characters. This latter appears to me identical with that figured from an imperfect specimen in the paper on the Glenshira Sand. A careful comparison of many frustules seems to confirm this opinion. The Clyde form is accurately described as follows:—” Surface of the disc marked with distant and irregularly radiate lines of rather large, round, distant cells or granules. The rays are distinctly marked towards the margin, but somewhat confused towards the centre. Puiicta or granules larger towards the centre than at the margin.” In the other form the rays are distinctly marked through the entire length, some of them reaching the centre, others terminating at some distance from it, and others extending but a short distance from the margin. The granules forming the rays are considerably smaller than those of the other species referred to, and the central ones are scarcely larger than those at the margin.

For these reasons I consider the two forms should be regarded as distinct species, and suggest thathenceforth the name Coscinodiscus Gregorianus should be given to the form found by Dr. Gregory in the Glenshira Sand.

I now proceed to mention a fact deserving of special attention, namely, this—that Tessella Interrupta, Eupleuria Pulchella, and forms belonging to the genera Hyalodiscus and Omphalopelta, have been met with in this collection. These species have been discovered in distant parts of the world, but, so far as I can learn, have not hitherto found a place in the list of British diatoms.

But the number of forms which, so far as I have been able to ascertain from the sources of information available to me, have not been hitherto described, constitutes the most interesting feature of this valuable collection. Some of these I shall hold over for further examination, and now submit to your consideration a few of these new forms, with their descriptive characters.

Navicula Hibernica, n. sp., O’M., Pl. V, fig. 1.—Broadly elliptical; length ·0041, breadth ·0024; striæ very fine, confined to a narrow marginal band; parallel to the median line there is a broad band without striæ, linear, interrupted at the central nodule, constricted towards the ends, and rounded; the central portion of the valve is granulated. This pretty form is closely allied to Navicula indica, Grev., but has not the mammiform apices nor the lyrate blank space of that beautiful species.

Navicula pellucida, n. sp., O’M., fig. 2.—Length ·0036, breadth ·0013; constricted; striæ very fine, confined to a very narrow marginal band, shorter towards the ends and the central constriction; the inner part of the valve smooth, pellucid; at either side of the median line divided into two compartments by a longitudinal curved line; in front view constricted, marked at the centre and ends by beadlike nodules.

Navicula denticùlata, n. sp., O’M., fig. 3.—Length of valve ·034, breadth·0013; deeply constricted; striæ costate rather than moniliform, marginal, with a narrow, striate, longitudinal band close to the median line; the interspaces blank. Properly this form belongs to the Pinnulariæ; but although the reasons assigned for merging the latter family in the N aviculæ seem scarcely satisfactory, I feel disposed to fall in with the tendency in this direction when the form presents the general characteristics of the Naviculæ. This species bears a striking resemblance to Navicula Egyptiaca, described by the late lamented Dr. Greville in the last number of the ‘Microscopical Journal.’ Navicula denticulata, however, is distinguished from that just referred to by the following characters:—It is much shorter and broader, the marginal striæ are longer, the central striæ are nearer to the median line, and continuous instead of being interrupted towards the central nodule as in the case of Navicula Egyptiaca.

Navicula Wrightii, n. sp., O’M., fig. 4.—Valves broadly elliptical; length ‘0041, breadth·024; striæ fine, marginal; there is a broad band at either side of the median line, linear, interrupted towards the central nodule, slightly constricted as it approaches the marginal band of striæ, and then expanding towards the apex, which is mammiform. This longitudinal band is destitute of striæ. On first inspection this form is liable to be mistaken for Navicula Hennedyi, but it is soon distinguished from it, not only by its mammiform apices and the spathulate extremities of the longitudinal median band, but also by the fact that in the present species this band is blank, while in Navicula Hennedyi it is striate, A variety of this species is described in fig. 4 B, much smaller than the other, and having the sides nearly parallel. Length ·0030, breadth ·0014.

Navicula amphoroides, u. sp., O’M., fig. 5.—Valve elliptical, narrow; length ·032, breadth ·0014. In this form the central nodule is depressed, and the median line waved; striæ moniliform, in the middle approaching the central nodule, and becoming gradually shorter towards the extremities.

Pinnularia Arraniensis, n. sp., O’M., fig. 6.—Valve broadly elliptical; length ·0030, breadth ·0017; striæ coarse, distinctly costate, not reaching the median line. In some aspects this form resembles Navicula Smithii and Navicula cestiva, Donk., but differs from the former by its distinctly costate striæ, and from the latter by the coarseness of its striæ, as also by the fact that it is much broader in proportion to its length than N. œstiva.

Pinnularia divaricata, n. sp., O’M., fig. 7.—Broadly elliptical, costate, length ·0058, breadth ·036; the ends slightly produced and rounded; the central space large, with an outline resembling the vertebra of a fish. Through this space there runs a well-marked median line, very fine at the outward extremity, and becoming broader towards the centre, at some little distance from which it terminates in a small bulb. The costæ are arranged concentrically with the apex at either end for about one third the length of the frustule, while those in the intermediate portion spring from the margin of the central nodule. The central costa runs at right angles with the longitudinal axis, and those at either side radiate towards it more and more as the distance from this line increases. The costæ in the central part of the valve are furcate. In some the furcation appears near the outer margin of the valve, in others near the central nodule; some few are bifurcate. It is worthy of notice that in some aspects the costæ appear as if they were slightly notched by longitudinal lines, which, though they produce a furrow, do not sink so deeply as to give a moniliform character to the sculpture of the valve.

Pinnularia constricta, n. sp., O’M., fig. 8.—Valve elliptical ; length·044, breadth ·0014; central nodule depressed; costæ distant, nearly reaching the median line, except at the central nodule; in front view constricted, linear.

Pinnularia forficula, n. sp., O’M., fig. 9.—Valve broadly elliptical; length ·0021, breadth ·0014. In the middle is a blank space, curved, constricted at the central nodule, and towards the apices from each side converging to a point. The striæ are distinctly costate, and longer at the middle than towards the apex. This form in its outline closely resembles Naυicula Smithii, var. δ, suborBIcularis, described by Gregory in his paper on the Diatomaceæ of the Clyde, but is distinguished from it by its costate striæ.

Surirella pulcherrima, n. sp., O’M., fig. 10.—Length ·0046, breadth ·037; broadly elliptical, ends symmetrical and nearly lanceolate; the border narrow; the central area wide, elliptico-lanceolate, and striate at the margin; canaliculi about fifteen on each side, at first narrow, then expanding towards the outward margin, the narrow part short and robust; alæ conspicuous.

Surirella gracilis, n. sp., O’M., fig. 11.—Length ·055, breadth ·0037; ends symmetrical and broadly rounded; canaliculi about twenty-four on either side, slightly radiate, narrow at first, and then expanding towards the outward margin, the narrow portion long, the expanded part first rounded and at a short distance from the junction slightly constricted, and gradually enlarging till it approaches the margin, where it terminates in a rounded end, separated from the next one by a very small space; outer margin finely striate, as is also the margin of the central area, which is elliptical; alæ not conspicuous.