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Fig. 28. Fig. 27. Rhynconella Laura.- Dorsal aspect. Fis. 27. The same; ventral aspect.
Fig. 28. Side view,
Description.—Ovate ; greatest width at about one-third of the length from the front; sides gently arched from the beak to the line of the greatest width ; front angles narrowly rounded; both valves convex, sometimes rather gibbous. Ventral valve most elevated a little above the middle; umbo rather obtusely rounded, not very prominent; beak short, obtuse, closely incurved, in contact with the umbo of dorsal valve; a broad mesial sinus which usually becomes obsolete at one half the length, but can be sometimes traced nearly to the beak. Dorsal valve with a mesial fold, corresponding with the sinus of the ventral valve in length.
Surface with about seventeen rather large rounded obscure slightly elevated ribs, of which there are four or five in the mesial sinus, and five or six on the mesial fold. A few squamose rings of growth.
Length of large specimen twelve lines; width eleven lines. Another individual from the same locality is nine lines long and ten wide.
Locality and formation.-Bosanquet. Hamilton Shales.
Genus ATHYRIS.-McCoy. There is much difference of opinion as to the propriety of retaining this generic name. It implies that the shells have no foramen in the ventral valve, and yet many species are placed in the genus which have the beak distinctly perforated. Some palæontologists are, therefore, in favor of using De Orbigny's appellation Spirigera, instead of Athyris. Nearly all of the Silurian species, and some of those from the Devonian rocks, have the beak so strongly incurved, that no foramen can be seen. For such, at least, the name Athyris does not appear to be very inappropriate. Mr. Davidson still retains it, not
only for those which have the foramen concealed, but also for those with it open. It appears probable that the genus will sooner or later be sub-divided, and in that case Athyris might be retained for the species with closely incurved beak, and Spirigera for some of the others. I shall give some account of the generic characters of this group of shells in another article. The following species are placed in the genus provisionally.
Fig. 32.- Dorsal view of a smaller specimen. Description.— Nearly smooth, ovate or sub-rhomboidal, greatest width, about the middle, a short linguiform projection in the middle of the front margin, both valves convex, ventral valve the larger, with its beak strongly incurved. Length from one to two inches; width equal to, a little less or a little greater than the length. The ordinary size is about one inch and a half in length.
The ventral valve is strongly and smoothly convex, the outline evenly arched from beak to front, more abruptly curved above than below, the umbo prominent, the beak rather small, neatly rounded at the sides, and closely incurved. The linguiform projection in the middle of the front margin, is often a simple extension of a portion of the shell, without a sinus, but occasionally there is either a short, shallow depression, or a narrow rounded mesial fold, which seldom, however, extends towards the beak more than three or four lines.
The upper half of the dorsal valve is sometimes evenly convex, but in general an indistinct, more or less broadly rounded carination, can be traced from the umbo along the middle to the front, where it becomes abruptly elevated into a short, prominent, rounded fold, which extends into the linguiform projection. On each side of the median line, this valve is gently convex, and often exhibits a rather fat slope to the lateral margins. The beak is strongly incurved, and appears to be deeply buried in the cavity beneath the umbo of the ventral valve.
If a line be drawn across the shell at mid-length, and another at one-fourth the length from the front, the greatest width will be found to range between the two. Many of the specimens are obtusely angular at the sides, and in such the margins above and below the angles are somewhat straight, the upper two sides converging to the beak, and the lower two to the linguiform extension in front, giving to the shell a rhomboidal instead of an ovate outline.
At first sight, the surface appears to be smooth, with a few concentric squamose lines of growth. On closer examination, numerous indistinct, radiating lines, may be seen. Of these, there are from two to four in the width of one line, and they sometimes appear to lie beneath the surface of the shell. In very perfect specimens, the surface exhibits fine concentric striæ, from ten to fifteen in the width of one line, and these are most distinct towards the front margin.
Beneath the beak of the detached ventral valves, there is a wide, triangular foramen, not visihle when the valves are united. The inside of the beak is entirely hollowed out into a deep pit or channel, which opens directly into the cavity of the shell. The impressions of the divaricator muscles occupy part of a sub-triangular space, the upper angle of which is situated just where the excavation beneath the beak
opens out into the visceral cavity. The lower side of this space is nearly straight, and the two lower angles rounded. The lateral margins of the space are usually sub-parallel in the lower half, while in the upper half they approach each other, and meet above to form the upper angle. In some specimens the space is more nearly triangular, and it would appear, therefore, that its form is a little variable. The length of the space is about one-third the whole length of the ventral valve, and its width at the lower margin a little less than its length. The lower margin is situated a little above a line drawn across the shell at mid-length. The lower three-fourths of the space is striated longitudinally, and divided into two equal portions by an obscure
On each side, at the base of the foramen, there is a short, stout tooth. The dental plates below the teeth extend but a short distance into the visceral cavity, when, becoming suddenly much diminished in height, they form a low ridge along the upper margin of the muscular space. The upper part of the muscular space is deeply excavated into the substance of the shell, which is very thick and solid in the rostral half.
I have not seen the interior of the dorsal valve.
Externally this species resembles A. tumida (Dalman,) but the muscular impressions in the interior of the ventral valve are widely different in the two species.
Locality and formation.-Rama's farm, near Port Colborne, and at many places in the County of Haldimand. Corniferous Limestone. Specimens, with the valves united, are rare, but the upper part of the ventral valve, with the umbo and beak preserved, is not uncommon.
Collectors.-A. Murray, J. De Cew, E. Billings.
ATHYRIS MAIA.-N. Sp. Description.-Smooth, ovate, or sub-rhomboidal. Ventral valve strongly convex, most gibbous in the upper half; umbo prominent, large, giving to the shell, on a side view, somewhat of the aspect of a Pentamerus ; beak strongly incurved, but not touching the surface of the dorsal valve ; a shallow, concave mesial sinus, extending from the front all the way to the beak. Dorsal valve moderately convex, with a convex mesial fold, which becomes obsolete near the beak. Length, from one inch to one inch and a half. The proportional width is variable. In some specimens it is exactly equal to the length, but in
others it is either a little greater or a little less. Greatest width, about the middle, or a little in front of the middle, at which point the rhomboidal specimens are angular, but in the more oval forms, gently convex.
The following characters may be more particularly noted :
The umbo of the ventral valve is rather large and prominent, the beak well defined, strongly incurved, but in general not in contact with the umbo of the dorsal valve; a short false area beneath the beak. The mesial sinus is shallow, evenly rounded in the bottom, or sometimes with an indistinct fold along the middle. Its width at the front margin is somewhat variable, but is usually about five lines, and it becomes gradually narrower and shallower upwards, but is more or less distinctly visible quite to the beak. On a side view, the outline of this valve presents a continuous curve, most abrupt in the upper half, the greatest elevation being at about one-third the distance from the beak to the extremity of the linguiform extension in front.
The dorsal valve has a smooth, rounded fold, extending almost to the beak, but is otherwise pretty evenly convex. It appears to possess a straight hinge-line, the length of which is greater than half the whole width of the shell; the beak small, not much incurved. The greatest convexity of this valve is about the middle, and on a side view the outline, in consequence of the elevation of the mesial fold, continues at the same height, and somewhat parallel to the lateral margin until it reaches the front.
The linguiform projection is sometimes considerably extended, and the shell has then a rounded, rhomboidal form, but in other specimens this part of the shell is truncated, and a fifth side, situated in the front margin, is thus formed.