characters, as also its position, which is not constant, being sometimes nearer one extremity than the other. 10 lc 000 BAIRDIA PLEBEIA, Reuss. Pl. IX., figs. 1 and 2, woodcuts 1a, 16, 1c. B. plebeia, Reuss. "Jahresbericht der Wetterau. Gesell. 1854," p. 67. Kirkby, "Ann. Nat. His.," 3rd ser., vol. ii., p. 324, pl. X., figs. 1 and 2. Length, 1-13th of an inch; height, 1-30th of an inch. Carapace subdeltoid, rather inflated centrally. Surface smooth. Dorsal margin prominently convex; posterodorsal region sloping abruptly, slightly concave; anterodorsal region sloping more gradually, straight, or slightly concave. Ventral margin straight or slightly incurved, more or less convex towards each extremity. Anterior extremity obliquely rounded, and rather produced. Posterior extremity acute, rostrated. Lateral contour regularly lenticular; greatest diameter (of lateral contour) two-thirds of the height. Flange of ventral margin sub-central. Dorsal overlap strong. B. plebeia enjoys a great latitude of form. Figs. 1 and 2 represent what are considered to be type-specimens. Several of the most important varieties will be separately noticed, and their differences pointed out. The modification of certain specific characters in the most aberrant varieties induced me at first to de scribe them as distinct species. But on a careful study of a very full series of specimens, I have reduced them to their more appropriate position of varieties. In the correction of these false determinations, I must acknowledge the kind assistance of Mr. Jones. Though the surface of this species is described as being smooth, Mr. Jones informs me that he has seen traces of pittings on some specimens from Kamsdorf, Germany; he has also observed similar traces on examples of B. ampla, from Hampole, Yorkshire. When Mr. T. R. Jones described the first-discovered specimens of this species, he referred them to the B. curta of M'Coy— a species found in the lower carboniferous rocks of Ireland. Subsequently Dr. Reuss met with more typical specimens in Germany, which he described, and named Bairdia plebeia. In the "Annals Nat. Hist." of last year, I identified a group of closely related varieties with Dr. Reuss's species, to which I at the same time referred the specimens described by Mr. Jones. However, on afterwards corresponding with the latter gentleman on the subject, I found that he was still inclined to refer all these specimens to B. curta. After some explanation on his part, and further examination on my own, I was disposed to come to the same conclusion. In arriving at this opinion, both Mr. Jones and myself relied upon the accuracy of Professor M'Coy's figures of B. curta, for up to this time we had not been able to examine specimens of that species from carboniferous strata. Fortunately, however, before the publication of these remarks, Mr. Jones received the type-specimen of M'Coy, which, when he had carefully extracted it from its matrix, so as to show its true form, proved to be a species distinct from the Permian examples. The figure given by M'Coy ("Syn. Char. Carb. Foss.," Ireland, pl. 23, fig. 6) does not represent the exact outline of the specimen, but as it appeared when partly imbedded in matrix. In B. curta, the anterior extremity is acute, and differs little from the posterior extremity, except in being less produced. In B. plebeia, the same feature is always comparatively broad, and more or less blunt; or, to speak more precisely, the gradual antero-dorsal slope, and the quick upturn of the angle, conjoin in giving this extremity what Mr. Jones graphically terms a "cheese-knife-like form," which holds good, more or less, in all the Permian varieties. It is the marked difference of this feature from its equivalent in B. curta, upon which Mr. Jones and myself chiefly rely, in considering B. plebeia to be a distinct species. B. plebeia occurs plentifully in some portions of the shelllimestone at Tunstall Hill, also rarely at Humbleton Hill, and not unfrequently in the upper limestone at Byers' Quarry. It is found in the Unter Zechstein at Bleichenbach, Selters, Gera, and near Saalfeld,* in Germany. B. PLEBEIA, var. CAUDATA. Pl. IX., figs. 10, 9, and 12; and woodcuts 2, 3, and 4. B. mucronata, Kirkby. "Ann. Nat. His." 3rd ser., vol. ii., pp. 327-328, pl. X., figs. 9-10. (Not B. mucronata, Reuss.) Length, 1-20th of an inch; height, 1-45th of an inch. Carapace sub-cuneiform; antero-dorsal region sloping more gradually than in plebeia proper, and forming a sharp angle, with antero-ventral curve, whieh forms a bold, convex sweep to the ventral margin; posterior extremity produced, acutely pointed, and curving upwards; lateral contour lenticular, having the extremities more acute than in type specimens. Var. caudata exhibits the broad cheese-knife-like anteroventral curve in its greatest development. The length and acuteness of its posterior extremity is also greater than in any other variety of this species. The peculiarity of these features give to it a delicacy of outline which is unequalled among the Permian Entomostraca. In classing this form as a variety of B. plebeia, I of course cancel my former identification of it with B. mucronata, Reuss, Dr. Richter, in the "Zeitschr. deutsch. geol. Gesell.," 1855, figured and described some varieties of B. plebeia, namely, B. drupacea (pl. xxvi., figs. 10-11); B. geinitziana (fig. 12); B. curta (figs. 13-15). On Mr. Jones addressing Dr. Richter respecting the specimens found at Saalfeld, he was immediately favoured by a suite of specimens and a letter from Dr. Richter, who stated his opinion that his B. drupacea, and B. geinitziana, were certainly varieties of what he termed B. curta. The last appears to be a variety of B. plebeia. which, in all probability, is quite distinct from the present species. B. PLEBEIA, var. AMYGDALINA. Pl. IX., fig. 11; woodcut 5. B. mucronata?, Kirkby. "Ann. Nat. His.," 3rd ser., vol. ii., p. 327, pl. X., fig. 11. mucronata, Reuss.) (Not B. Carapace sub-ovate; narrow and rounded in front; obliquely acute behind; antero-dorsal region gently sloping; postero-dorsal, with a rapid slope and slight incurvature; ventral margin straight, with a bold curve anteriorly, and a slight curve upwards behind; lateral contour shows compressed extremities, especially in front, and a strong convexity a little behind the middle. B. PLEBEIA, var. ELONGATA. Pl. IX., fig. 4; woodcut 6. B. plebeia, var. elongata. "Ann. Nat. His.," 3rd ser., vol. ii., p. 325, pl. X., fig. 4. Carapace elongate; postero and antero-dorsal regions sloping gradually; anterior extremity somewhat produced; lateral contour more compressed than in B. plebeia proper. The present variety is B. plebeia proper elongated and compressed; curtail its length and increase its width, and we have the typical form of the species. B. plebeia, var. elongata, is a common form at Tunstall Hill; it also occurs in a magnesian limestone at Hampole, Yorkshire. B. PLEBEIA, var. NEPTUNI. Pl. IX., fig. 5; woodcut 7. B. plebeia, var. Neptuni. "Ann. Nat. His.," 3rd ser., vol. ii., p. 325, pl. X., fig. 5. Var. Neptuni has an elongate carapace, with the greatest convexity of dorsal margin very posteriorly situate; it consequently possesses an abrupt posterior slope, and a very gentle anterior one; its posterior extremity is broadly beaked, and its lateral contour somewhat compressed. Besides occurring at Tunstall Hill, I have also met with this variety at Hampole, Yorkshire. VOL. IV. PT. II. T |