« EelmineJätka »
Description.--Sub-orbicular or sub-quadrate ; length about eightninths of the width ; greatest width, usually a little in front of the middle; length of hinge line, one half to two-thirds the width of the shell ; cardinal extremities rounded ; sides in most specimens somewhat straight, often sufficiently carved to give a circular aspect to the shell ; front angles obtusely rounded; front margin in general broadly convex, sometimes with a small space in the middle nearly straight. Dorsal valve of a medium convexity, most elevated about the middle ; the outline forming a uniform arch from the depressed beak to the front margin ; the slope from the umbo towards the cardinal angles, gently concave; sometimes a barely perceptible mesial depression, commencing in a point at the beak, and becoming obsolete at one half or two-thirds the length ; area small, lying in the plane of the lateral margins ; beak minute, forming a small triangular projection, rising scarcely one-fourth of a line above the edge of the area, the point not incurved over, but situated in the plane of the area. Ventral valve moderately convex, most elevated at between one-fourth and one-third the length from the beak, thence descending with a somewhat flat or gently convex slope, to the front and sides, and with a more sudden and somewhat concave slope to the hinge line and cardinal angles ; the umbo small, prominent, neatly defined, terminating in a small rounded beak, which is incurved so as to overhang the edge of the area, either not at all or scarcely one-tenth of a line ; area triangular, about onefourth larger than that of the dorsal valve, forming an angle of about 105o with the plane of the lateral margin. The foramen not observed, but appears to be wider than high. On looking at the dorsal valve in a direction perpendicular to the plane of the shell, the small rounded umbo of the ventral valve can be seen rising about one-third of a line above the dorsal beak.
Surface with small sub-angular radiating ridges, of nearly a uniform size, from eight to ten in the width of three lines, increasing by bifurcation, strongly curved outwards to the upper part of the sides and cardinal angles, the intervening grooves sub-angular in the bottom, and equal to the ridges in width. In very perfect specimens, very fine concentric sub-lamellar concentric striæ are visible, seven or eight to one line. In certain conditions of preservation also, the radiating ridges are seen to be sub-tubular, and exhibit numerous small oral or circular openings on their edges, each about the eighth or tenth of a line in width, and from one-fourth to two-thirds of a line distant from each other.
Width of largest specimen examined, eighteen lines; length, sixteen lines; thickness or depth of both valves, seven lines; height of area of ventral valve at the beak, one line; area of dorsal valve, fourfifths of a line ; distance between the beaks, one line ; length of hingeline, ten lines. The most common size appears to be one inch in width. The beak of the ventral valve is incurved, so that it would touch a plane projected horizontally through the valve, at one-half the depth of the cavity.
In some specimens the ventral valve has a faint, barely perceptible mesial fold, extending from the umbo towards the front.
This species is allied to 0. Vanuxemi, but is more coarsely striated. It may be identical with one of the other species described in the Report of the Regents of the New York University, but as it is impossible to identify it with any of the descriptions, I propose to name it as above.
Locality and formation.—Township of Walpole. Corniferous Limestone.
Collector.-J. De Cew.
ORTHIS VANUXEMI.-Hall. Tenth Annual Report of the Regents of
the University of the State of New York, p. 135, 1857.
This species is closely allied to 0. Livia, but is more nearly a perfect ellipse, or more nearly circular, and has about fifteen radiating striæ in the width of three lines. Its width is from nine to eighteen lines, and its length about one-sixth or one-seventh less than its width.
It occurs in the Hamilton Shales, in the Township of Bosanquet.
Description.—In this species the body of the shell, excluding the beaks, is transversely sub-oval ; from the beaks the sides diverge at an angle of about 110°, and are nearly straight, or gently concave, for about one-third the length of the shell, below which they are regularly rounded; front margin broadly rounded, nearly straight in the middle for the width of the mesial fold. On a side view the outline is obliquely sub-conical, the base obtusely rounded, the lower three-fourths of the ventral valve straight, forming an angle of about 100° with the ventral half of the base; the dorsal contour gently arched, and the dorsal half of the base rounded. The mesial sinus of the ventral valve commences at about mid-length, and increases only slightly in depth, until the front margin is reached, when the middle portion of the shell, for the width of the sinus, is rather suddenly bent towards the dorsal valve, with an abruptly rounded curre, and forms about three-fourths of the depth of the base. There are five angular ribs in the sinus, their extremities deeply forked to receive the projecting points of the grooves between the ribs of the dorsal valve ; on each side of the sinus there are six principal ribs, their extremities a little turned upwards ; above these, three or four smaller ones. The upper part of this valve is narrowly convex, with a prominent umbo, and incurved beak; in the lower half, a little concave towards the lateral margins, owing to the elevation of the extremities of the ribs. Dorsal valve with all the central region depressed convex, the margin of the shell on each side of the mesial fold in front abruptly bent towards the ventral valve ; the umbo obtuse, divided along the middle for a short distance by a narrow, barely perceptible depression, the beak strongly incurved under that of the ventral valve. The mesial fold dies out at two-thirds the length, the shell (of the fold) at the lower extremity bent towards the ventral valve at an obtuse rounded angle, and extending about one-fourth across the base. On the mesial fold there are six ribs, the middle four most
prominent; on each side of the fold there are also six, their extremities strongly curved towards the ventral valve.
I have seen no specimens with the true surface of the shell preserved.
Length of specimen upon which the above description is founded, nine lines, greatest width at about one-third the Tength from the front, ten lines : depth of both valves at the front, six lines ; width of mesial fold at front, five lines, and of the sinus, five lines and a half.
Judging from the appearance of several imperfect specimens, the depth of both valves at the front must be very variable.
I have seen some small specimens from four to six lines in length, with a beak nearly erect. These, I think, are the young of R. Tethys.
Locality and formation.--County of Haldimand. Corniferous Limestone.
Collector.-J. De Cew.
RHYNCONELLA MEDEA.--N. Sp. Description.--Oval or sub-triangular, body of shell, excluding the beaks, transversely sub-elliptical ; greatest width a little below the middle ; apical angle, about 100o; both valves rather convex. Ventral valve with a neatly defined, rounded umbo, and closely incurved beak; a wide, shallow, concave, mesial sinus, which becomes obsolete at about two-thirds the length from the front. Dorsal valve, with a broad, depressed, convex, mesial fold, extending two-thirds the length of the shell, umbo rather prominent, obtusely rounded, beak incurved beneath that of the ventral valve. Surface with between thirty and thirty-five small sub-angular ribs on each valve ; ten on the mesial fold, and nine in the sinus.
Length, eleven lines. Width, twelve lines. Depth, seven lines; width of sinus at front margin, six lines.
The specimen is a little distorted towards the front, so that all the details of the outline cannot be given. The sides diverge from the beak at an angle of about 100°, and are straight for half the length of the shell. They then appear to be somewhat narrowly, but regularly, curved round to the front, which is also, I think, broadly rounded.
Locality and formation.-Township of Rainham, Concession 3, Lot No. 2.
Collector.-J. De Cew.
Description.—Shell small, apical angle varying from about 70°, in very small specimens, to 105° in the large ones ; sides straight in the upper half, regularly curved in the lower half; front broadly rounded with usually a portion in the middle straight, or even slightly concave ; valves about equally convex. Ventral valve with a sinus which gradually dies out at one half, or a little more, of the length from the front; beak acute, much elevated, slightly incurved; three simple acutely angular ribs in the mesial sinus, and six or seven on each side. Dorsal valve a little more strongly convex than the ventral valve ; the front of the mesial fold elevated so that on the side view the base of the shell is a nearly straight line almost at a right angle with the lower part of the outline of the valve ; umbo rounded with a faint mesial depression ; beak incurved into the cavity of the ventral valve ; surface with four ribs on the fold, and six or seven on each side.
Length of the largest specimen examined, four lines ; width, four and one-fourth lines ; depth, two and a half lines; width of the sinus, nearly two lines; apical angle, 102°.
Another specinen is four lines wide, three and a half in length, two in depth, sinus, two lines, and apical angle 1050.
A third is two lines and three-fourths in length, and the same in breadth ; depth, one line and three-fourths; apical angle, 889. The sinus is distinct but not deep.
Specimens less than two lines in length exhibit scarcely a trace of a sinus, and have the apex more acute than any of the above-mentioned.
This species closely resembles the ordinary Lower Silurian forms, such as R. plena, and young individuals of R. increbescens.
Locality and formation.- Near Woodstock. Corniferous Lime. stone.