The variation of the posterior extremity is of most importance, as in some cases the slight modification which it undergoes causes it to assume the form of the anterior.

One of the most peculiar characters of this species is the curious marginal rims which bound its free margins, and form so important a feature in its ventral aspect. I have never observed more than two rims on each valve, except in one instance, which was a perfect specimen, having three rims on the right valve, with only two on the left. These rims very much remind one of the exfoliative dilatations of the margins which are seen in some species of Mollusca; but when the mode of growth of the Entomostraca is considered, such an idea is found to be untenable; for we must suppose that these animals, like their recent representatives, would increase in size by moulting, and not by marginal increment. The youngest specimens possess the rims in miniature; indeed, all stages of growth are characterised by them, though the older individuals have them most produced. One of my specimens (fig 11, pl. X.) appears to have several fine lines between the outer and inner rims and running parallel to them. The same portion of the valves are occasionally seen to be delicately reticulated after the pattern of the rest of the surface. (Fig. 5c, pl. X. A.)

The central area of the valves is generally very much produced, but more so in some specimens than in others. Sometimes its connection with the marginal portion of the valves is so abrupt as to cause it to appear like a great tubercle; at others it slopes more gradually towards the margin, and wears a less gibbose aspect: this is particularly the case in young specimens. The postero-dorsal region of this area is always the most prominent portion of the valve; and as the central portion of the dorsal region is at times rather depressed, both it and the anterodorsal angle have there a gibbose appearance. Such specimens assimilate to the K. (Cythere) Schrenkü of Keyserling, the equivalent regions of which are extremely gibbose. As the central area varies in prominence in different specimens, so do specimens vary in width, and that very considerably.

"Reise durch die Tundren der Samojeden, 1854," p. 112, taf. 4, fig. 37.

K. Permiana is not uncommon in the shell-limestone of Tunstall Hill, and in the upper Permian limestone of Byers' Quarry. K. Permiana seems to be nearly related to the Russian species K. (Cythere) sticta, Keyserling.* The latter form has rounder extremities, and its ventral margin more deeply sinuate, and it is apparently more compressed than the former. Some difference also exists in the ornamentation, which in K. sticta appears to be very regularly arranged. Both agree, however, in possessing two expanded rims on the free margin of each valve. K. Roessleri, Reuss,† of the Unter Zechstein, also corresponds in this respect, and will probably prove to be closely related.

The generic affinities of this species and of its congeners have been until lately involved in some obscurity. When first described by Mr. Jones, from specimens on the limestone slabs of Byers' Quarry, it was referred by him to the genus Dithyrocaris of Dr. Scouler. In placing it there, he admitted that it was questionable whether it in reality belonged to that group or not; for his own specimens were not well preserved, nor had the characters of the genus been properly defined by its author; but from the data offered by the imperfect materials in his hands, he thought it possible that it might have some affinity to it, and placed it in that genus provisionally.

Dithyrocaris was originally considered by Dr. Scouler to have a univalve carapace, like Apus and other single-valved Branchiopoda. He afterwards altered his views, supposing it to be bivalve, like Cypris, though differing from that genus in having caudal appendages, protruding from the valves. This opinion was held until 1843, when General Portlock described two new species from the slabs of the Lower Carboniferous rocks of Ireland, and proved that its carapace was univalve, as Dr. Scouler had at first supposed. General Portlock's description of these species, particularly of D. Colei, can leave no doubt of the correctness of his views in this respect, and clearly demonstrates that Dithyrocaris is a univalve Entomostracan- that is, sup

"Loc. cit," p. 112, taf. 4, fig. 38.

"Jahres. Wetterau. Gesell.” 1854, p. 70.

"Records of Science," Feb. 1835; and in a paper read before the British Association at Glasgow. See also Jones on Dithyrocaris, in Professor King's "Mon. Perm. Foss," ." p. 64.

posing Dr. Scouler's species belong to the same group, which be taken for granted until proved to the contrary.

may

All the examples of K. Permiana which came under the notice of Mr. Jones were, as stated before, more or less imperfect, so it may naturally be supposed that great difficulty would occur in attempting to determine the generic affinities of the species to which they belonged. It was evidently owing to this cause that Mr. Jones supposed that it might be a member of Dithyrocaris. It is from the perfect state of preservation of my specimens that I have been enabled to offer the preceding remarks in addition to those of Mr. Jones. And as some of the specimens have their valves united and in close juxtaposition, I have also been enabled to prove that the species is a bivalve Entomostracan, like Cythere, or rather Beyrichia, consequently that it has no affinity to Dithyrocaris, or to Ceratiocaris, to which Mr. Jones afterwards referred it, the closely fitting edges of its carapace, of course, excluding it from that genus.

German and Russian palæontologists have referred congeneric forms to Cythere; but, with the exception of being bivalve, they possess no characters to warrant their remaining in that genus.

REMARKS BY T. RUPERT JONES, F.G.S.

The specimens of Magnesian limestone with which Professor King supplied me in 1849, for the obtaining of Foraminifera and Entomostraca, to be described in the "Monograph of the Permian Fossils of England," yielded numerous casts and indifferently preserved valves of Entomostraca, which I could not place with the Cytheres, on account of their relative size, shape, and style of ornament, and which I erroneously referred to Dithyrocaris (seeing some apparent relation between them and D. tenuistriatus). Subsequently I suggested to Mr. Morris, when he was preparing the new edition of his "Catalogue," that these Permian Entomostraca should be removed from Dithyrocaris, and provisionally placed with the bivalvular Ceratiocaris, until we should know more about them. At this time Professor Reuss figured and described an evidently allied form, under the name of Cythere Roessleri, from the Lower Zechstein of Bleichenbach; and soon

afterwards Dr. Richter figured and described another, somewhat differing from the foregoing, under the same appellation, from the dark-grey limestone of the Lower Zechstein of the Roth Berg, near Saalfield, and recognised its probable affinity to the so-called Dithyrocaris Permiana.

The numerous fine specimens, however, obtained by Mr. Kirkby from the Magnesian limestone of Tunstall Hill enabled him, last year, to draw up an exact description of these interesting Entomostraca; but, thinking that I might have means for a wider comparison than he possessed, and knowing that I had already interested myself in the subject, he most courteously and considerately left to me the determination of the generic alliances; and liberally confided to me all his materials for examination. The result has been that, after a careful comparison of Silurian, Carboniferous, and Permian species of near alliances, I believe that we have, in these peculiar Permian forms, and in a related species from the Lower Carboniferous rocks of Scotland, a distinct generic type not far removed from Beyrichia. For this genus I propose the name of Kirkbya; for without Mr. Kirkby's industrious researches and acute observation, we should still have wanted the abundant and illustrative materials which we now possess for the natural history of this Permian species.

I had recognised in the figures and descriptions given by Reuss and Richter of their Cythere Roessleri an affinity to my so-called Dithyrocaris Permiana and D. glypta; and, in the autumn of 1857, Dr. Richter favoured me, at Saalfeld, with two specimens of the species which he had described as Cythere Roessleri (Reuss); but the finely-sculptured surface of the German specimens offered so strong a contrast to the bareness of the valves from Byers' Quarry, that I gave up the hope of finding more than a consanguinity between these peculiar English and German Entomostraca; and I trusted to be able some day to point out that they were neither Dithyrocaris, Ceratiocaris, nor Cythere.

The carapace-valves from Saalfeld had presented to me not only a finely reticulated surface (well figured by Richter), but also a small subcentral sunken oval spot, around the smooth edge of which the meshes of the superficial net-work had a more or less distinct concentric arrangement for a little space. Of this fea

ture, which is indicated in Richter's figure ("Zeitschrift," vol. vii., pl., 26, fig. 2.), I could not find clear evidence on the specimens which I possess from Byers' Quarry, although a trace of reticulation on one valve, and a slight subcentral circular mark on the casts of others, strengthened my belief that they might belong to the peculiar genus which comprehends the socalled Cythere Roessleri.

Mr. Kirkby's specimens from Tunstall Hill, occurring under the circumstances so well described by him (see above), enabled him to set these peculiar Beyrichia-like forms again before palaontologists, and this time with full and clear descriptions, accompanied by several sketches ("Annals of Natural History," 3 ser., vol. ii., pl. 11, figs. 5-13). Following Mr. Kirkby's suggestions, assisted by his notes, and carefully studying his numerous specimens, I am now enabled to show them in a lithograph executed by Mr. G. West, with their details claborated, and in comparison both with the German forms and with the specimens from Byers' Quarry.

Some of the features which I have lately recognized in Mr. Kirkby's specimens are alluded to above in his revised description of the species. I need only add the following remarks.

The sculpture of the surface of the valves is subject to considerable variation as to its distinctness. In the specimens which I have seen from the Zechstein of Saalfield and Gera, it is very distinct and uniform; the subcentral oval spot is well defined; and the inner border of the innermost of the ventral ridges is strongly marked by a line of small roundish pits (figs. 8a and 86; shown also by Richter). A faint trace of this linear punctation I have lately found in a specimen from Byers' Quarry. Not unfrequently the longitudinal boundaries of the hexagonal pits, on the surface of the valves, are strengthened (fig. 2a), and occasionally exaggerated (figs. 4 and 6), in which case we get the ridged variety, K. Permiana, var. glypta, which I named as a species in the "Monog. Perm. Foss." Sometimes a single ridge only is developed on the valve, as in fig. 5a. Reuss's C. Roessleri (fig. 9) is evidently one of those ridged varieties, and may be distinguished as K. Permiana, var. Roessleri. Richter's C.