Roessleri, with its uniformly sculptured surface, punctured ventral ridge, and well-marked oval spot, may be regarded as a separate variety (K. Permiana, var. Richteriana). I quite agree with Mr. Kirkby in his observations on the K. (Cythers) sticta, of Keyserling (see above, p. 132), which closely resembles the specimens referred to by Richter to K. (Cythere) Roessleri, of Reuss. Nor can I see, in Keyserling's figures, any specific differences between his C. Schrenki, C. sticta, and C. grapta; they are, probably, varieties of K. Permiana; and, until we can compare the specimens, we may recognise them as K. Permiana, var. Schrenki, sticta, and grapta. The other species of Kirkbya, with which I am acquainted, has a thick oblong carapace, with its ends nearly uniform in thickness with the central portion. Its ventral border is thick and doubly ridged. The surface of the valves is reticulated, and characterised by two somewhat semicircular ridges, one within the other, their ends touching the dorsal edge. The oval spot is nearly central, and placed at the curve of the inner and smaller ridge. The specimens are in the Hunterian Museum of the Royal College of Surgeons, of London, and were collected and presented to John Hunter by the Rev. Dr. Ure. the lower Carboniferous shales, near Glasgow. nominate this species Kirkbya Urei. They are from I propose to de The generic characters of Kirkbya may be stated as follows:Animal enclosed in a bivalved carapace; carapace equivalved; carapace-valves thick, oblong; anterior and posterior extremities more or less rounded; ends of the dorsal border somewhat acute; ventral border straight in the centre and curved at the ends. Valves usually wider at the caudal than at the cephalic extremity, convex, ridged strongly on the ventral border and at the ends; the ventral edge of one valve (dextral) slightly overlapping that of the other; surface impressed with a small subcentral oval hollow, and more or less ornamented with a reticulate sculpture and ridges. Hinge probably consisting of a simple adaptation of the dorsal edges and their union by membrane. In the general form, hingement, and mode of overlap, Kirkbya resembles Beyrichia; and K. Urei slightly approaches some Beyrichia (such as B. Bussacensis) in the setting on of its ridges. We also see in the figure of K. Permiana, var. Schrenki, and occasionally in some of the Durham specimens (see fig. 1a), a tendency to the bilobed condition of the valve, due to the subcentral dorsal furrow, so characteristic of the Leperditio. The oval spot, however, the hexagonal reticulation, the long stretching wrinkles and riblets, and the doubly and sometimes trebly ridged ventral margin, are characters not found in Beyrichia, and must be regarded as distinctive of a separate generic group. Lastly, it seems to me probable that the sunken oval spot on the valves may have reference to the place of attachment of the transverse muscle of the animal, and thus represent the variously patterned "muscle-spots" of other bivalved Entomostraca. CYTHERIDE. AMONGST the Entomostraca of the shell-limestone none are so plentiful as the Bairdiæ. Of these there are several forms, which somewhat resemble the Bairdia curta (M'Coy) of the lower carboniferous rocks of Ireland, and are not very dissimilar to the B. subdeltoidea (Münster) of the Cretaceous and Tertiary formations, and of existing seas. These Permian specimens vary considerably among themselves in outline and relative convexity. Several of them were described in the " Annals of Natural History," 3rd series, vol. ii., pp. 324, &c., under the specific names of B. plebeia (with three varieties-elongata, Neptuni, and compressa), B. ventricosa, B. Reussiana, B. Kingi, B. mucronata, B. Schaurothiana, B. Berniciensis, and B. rhomboidea. Three other Cytherida, from the shell-limestone, were also described at the same time as Bairdiæ, viz., B. reniformis, B. Jonesiana, and B. truncata, which, however, are regarded by my friend Mr. Jones as more likely belonging to another sub-genus. Having asked Mr. T. R. Jones for his opinion on the arrange VOL. IV. PT. II. S ment which I had proposed for these forms, as above indicated, and shown him all my materials, he recommends me to annul some of my determinations, and to group into the following species (as far as the Bairdiæ are concerned) the forms which I had sorted out as characteristic. 5 B. mucronata? Reuss. fig. 10.... 2. B. plebeia, var. caudata.. fig. 9...... fig. 12.... 4 B. fig. 11.... fig. 4...... fig. 5...... fig. 6...... 8 B. Reussiana, Kirkby. 6 B. plebeia, Reuss.var elongata 7 B. plebeia, Reuss. var Neptun 3. B. plebeia, var, amygdalina From his opportunities of studying the carapace-valves of both recent and fossil Bairdiæ, Mr. Jones appears to have reason for differing from me in the estimate made of the value of some features upon which I had much depended, namely, the relative gibbosity of the valves (shown in the lateral contour), and the amount of ventral overlap. More particularly, my friend's views are at variance with those advanced by me as to the sub-generic relations of what I termed Bairdia reniformis, B. Jonesiana, and B. truncata. These he believes to belong to Cytherideis, a subgenus established by him in 1856, for the reception of several Cytheres nearly approaching Bairdia in many respects. For the sake of easier comparison, and to illustrate the gradual development of varietal differences, it has been deemed advisable to introduce a series of carefully executed woodcut outlines of the Bairdiæ above referred to. CYTHERE, Müller. SUB-GENUS BAIRDIA (M'Coy). Jones' "Monograph of the Entomostraca of the Cretaceous Formation of England," p. 22; and "Monograph of the Tertian Entomostraca of England, p. 51. Bairdia (genus), M'Coy, 1844. "Syn. Char. Carb. Foss. of Ireland," p. 164. This group was instituted by Professor M'Coy, in 1844, for the reception of two species of Entomostraca, from the carboniferous rocks of Ireland, which he considered to differ generically from any existing genus. Mr. T. Rupert Jones, in his "Monograph of the Entomostraca of the Cretaceous Formation of England," 1848, proposed its adoption as a sub-genus of Cythere, giving a good definition of its sub-generic characters, which had scarcely been done by M'Coy. I attach to it a similar value in these remarks. Mr. Jones has given further illustrations of the sub-genus in his "Monograph of the Tertiary Entomostraca" (Palæontographical Society, 1856). Dr. Baird has placed it, as modified by Jones, among his synonyms of Cythere. He states that no anatomical difference exists in the animal of those recent species referred to it by Jones, from the animal of Cythere proper.* This is certainly a strong point in support of its near affinity with Cythere; but still, with its marked peculiarity of carapace, I think it may be fairly entitled to a distinctive term, to mark it from the typical forms of Cythere. At least, whether it really be a natural sub-genus or not supposing such to exist-it is a very convenient group in palæontology, as it serves to mark a number of fossil Entomostraca of peculiar character, which have repeatedly held no unimportant position in the economy of ancient faunas. The sub-generic characters of Bairdia have already been given by Mr. Jones. The Permian species add little that is new to them, though it may be well to recapitulate the sub-generic features of these forms. They all appear to be smooth; sometimes "Natural History of the British Entomostraca" (Ray Society), p. 163. the carapace is finely pitted, and occasionally hirsute in the recent state. The general form of the majority is somewhat triangular or subpentagonal, though species or varieties having a still more elongate contour occur. The posterior extremity of all is more pointed than the anterior, and it is generally rostrated. The anterior extremity is usually rounded, but sometimes subangulated, and also much wider than the posterior extremity. The ventral margin is always the straightest, though often somewhat sinuated. The dorsal margin is more or less convex centrally, and generally sinuated towards the posterior extremity. In lateral contour* they are lenticular, variously modified, being sometimes almost exactly lenticular; at others, by the extreme position of their greatest diameter, sub-cuneiform. Perhaps the most important generic character of Bairdia is its hingement, which is a lapping of the left valve over the right, along the dorsal-margin, and which was first noticed by Mr. Jones. In some Permian varieties, the folding of the valves is relatively great; and in most cases the folding is much greater in the centre than near the extremities. In general this character does not vary much in the Permian species. A central third of the ventral margin of the left valve overlaps an equivalent portion of the right, giving the contact-line of the ventral margins a sinuated appearance. The mode of its overlapping is this:-From each extremity, until approaching the central portion of the ventral margin, the extreme edges of each valve are somewhat produced, and lie against each other in close juxtaposition, like the valves of a conchifer; towards the centre, however, the edges become flattened and bent inwards, forming slight horizontal ledges or flanges, that of the right valve being rather smaller than the other which overlaps it. In no instance does the whole margin of one valve overlap that of the other; in no species is more than one-third overlapped, but sometimes less. The length of margin overlapped, and its width, may occasionally be of use as specific * The term "lateral contour," is used for the outline which is seen by viewing the carapace ventrally. "Ventral aspect," is used in similar instances by others; but, as it is chiefly the contour of the sides of each valve of the carapace that is referred to when using the term, I think the former better expresses the meaning attached to it. |