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longitudinal folds (obscure plications) on one or both sides, which become more strongly developed towards the aperture, and are very conspicuous in old shells; the right side is more expanded than the left, and in some well preserved specimens is nearly twice as wide. Aperture very oblique, rhomboidal or subtriangular, and the peristome sinuous.

SURFACE marked by fine, closely arranged, undulating striæ of growth, which are not lamellose.

This species is very well marked in its dorsal carina and rapidly expanding body-volution, which spreads always more on the right side. The surface, though distinctly striated, is close, and the shell compact; differing in this respect from some of the other species. Having examined more than a dozen individuals, from the length of less than half an inch to that of an inch and a half, I find that the characters mentioned are preserved in a marked degree in all. In the largest specimens, the aperture is a little more than an inch in its greatest diameter, and nearly equal to the height of the shell.

Formations and localities. In limestones of the Upper Helderberg group, Helderberg mountains; Williamsville, N. Y., and at Sandusky, Ohio; in the Hamilton group, at Eighteen-mile creek, Darien, Pavilion, Canandaigua and Seneca lakes.

PLATYCERAS (ORTHONYCHIA) ATTENUATUM..

PLATE III, FIGS. 1-6.

Platyceras attenuatum HALL. Descriptions of New Species of Fossils, etc., p. 2. 1861.

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Not Platyceras attenuatum, Meek: Proceedings Phila. Acad. Nat. Sci., p. 75. 1871.

Not P. dumosum v. attenuatum, Meek: Geol. Rep. Ohio: Palæontology, I. Expl. of plate 20, fig. 1.

SHELL elongate-ovate or conically subovate with a slender apex, the nucleus making about one valution or one and a half, below which the bodywhorl becomes rather abruptly inflated, and thence gradually expands to the aperture, which is very oblique-the anterior side of the peristome being much more extended.

SURFACE marked by crowded, undulating, concentric striæ and longitudinal, irregular and undefined folds, which vary greatly in different specimens: the latter becoming more distinctly marked as plications near the aperture. Peristome sinuous, with numerous indentations corresponding to the folds upon the surface.

In many specimens the surface is marked by abrupt undulations without distinct folds or with the folds obscurely developed. Length of shell about one inch, with the greatest diameter a little less than half an inch.

This species is distinguished by the abrupt contraction of the upper part of the shell at the beginning of the body-whorl or just below the curvature: while the apex, consisting of scarcely more than a single minute volution, is abruptly contracted, and proportionally more slender than in most other species. It somewhat resembles the P. clavatum of the Lower Helderberg group.

Formation and localities. In the shales of the Hamilton group on the shores of Seneca and Cayuga lakes, Geneseo, Moscow, and other places in Western New York.

PLATYCERAS AURICULATUM.

PLATE III, FIGS. 8-10.

Platyceras auriculatum HALL. Illustrations of Devonian Fossils: Gasteropoda, pl. 3, figs. 8-10. 1876. SHELL Subovoid, extremely arcuate, the nucleus making little more than one volution, beyond which the shell at first gradually expands, and finally becomes ventricose below, being much more expanded on the right side. The back of the shell, to the end of about the second volution, shows the existence of a deep dorsal sinus, which at this point becomes filled, and the margin of the shell projects, while the peristome has become indented by numerous shallow sinuosities, appearing on the surface of the bodyvolution as sharp carinæ or subdued plications; each one of which indicates the degree of sinuosity in the margin of the shell. The expansion on the right side, beginning as described, is marked by three prominent carinæ, while the remaining portion of the surface is traversed by smaller and less distinct elevations, of which five occupy the dorsum towards the aperture.

SURFACE marked by fine closely arranged concentric striæ, which are crowded in fascicles from irregular growth, and are distinctly undulated in crossing the folds of the shell. In parts of the surface, where well preserved, fine revolving striæ are seen. The aperture is subrhomboidal, and the peris-tome sinuous.

This form possesses many characters in common with P. carinatum, but there is no development of a carina upon the dorsum, though we may consider that this feature is represented in the several low folds which are all in the line of the deep sinus which existed in the early growth of the shell, and that the single carina is represented by the four or five smaller folds. The posterior margin of the peristome is likewise in close proximity to the spire, as in P. carinatum. In other respects, it presents some characters in common with P. symmetricum. Formation and locality. In a calcareous band in the shales of the Hamilton group at Darien, N. Y.

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PLATYCERAS THETIS.

PLATE III, FIGS. 11-16.

Platyceras Thetis HALL. Descriptions of New Species of Fossils, etc., p. 4. 1861.
Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 32. 1862.
Illustrations of Devonian Fossils: Gasteropoda, pl. 3. 1876.

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SHELL obliquely arcuate from the base, with the apex incurved, the nucleus making barely more than a single minute volution; gradually expanding from the apex to near the aperture, which is sometimes more abruptly spreading. The back of the body-whorl is prominent, and a little flattened on the left side; while the right side, from one-third to one-half the length, is sometimes marked by two or three longitudinal folds, and often by more numerous, finer plications. Aperture a little oblique, nearly round or subquadrangular, with the peristome sinuous.

SURFACE marked by fine, closely arranged lamellose striæ, which are abruptly undulated on all parts of the body of the shell.

In many, and perhaps nearly all specimens, the body of the shell, along a line a little to the left of the dorsum, is marked by an abrupt curvature of the

striæ, indicating a notch in the margin of the peristome. This line is sometimes marked by a narrow prominent band, not unlike that of PLEuroto

MARIA.

This species differ from P. attenuatum, in being arcuate from the base, in the gradual attenuation towards the apex, and the more closely incurved nucleus and less abrupt expansion.

An unusual form (fig. 30, pl. 3) has been referred with doubt to this species. It has the general form and expression of P. Thetis, but is much larger than the prevailing forms, and preserves several protuberances on the surface which have the appearance of spine-bases. The surface preserves the fine revolving striæ crossing the concentric ones.

Formations and localities. In shales of the Hamilton group, at Moscow, York, and Ontario county; also in limestone of the Upper Helderberg group, Albany county, N. Y.

PLATYCERAS SYMMETRICUM.

PLATE III, FIGS. 17-25.

Platyceras symmetricum, HALL. Descriptions of New Species of Fossils, etc., p. 6. 1861.
Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 34. 1862.
Illustrations of Devonian Fossils: Gasteropoda, pl. 3. 1876.

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SHELL elongate, subovoid, arcuate, incurved nearly in the same plane; nucleus minute, the spire making about one volution or one and a half, when the body-whorl becomes free and rapidly, or somewhat abruptly, expanded; spreading about equally on the two sides of the dorsum, which is more prominent and sometimes marked by a ridge. Aperture oblique, subquadrate or rhomboidal; margin of the peristome sinuate, and on the posterior side distant from the spire.

SURFACE marked by concentric undulating striæ, and longitudinally by obscure interrupted ridges, which, on some parts of the older shells, become regular and uniform, with a narrow groove between.

This species is well marked by the equilateral expansion on each side of the dorsum, and by the volution of the apex being nearly in the same plane. The

posterior margin of the aperture is widely separated from the preceding volution. The longitudinal ridges are more strongly marked, and of a different character from those of characteristic specimens of P. bucculentum; it is not, however, improbable that we may find intermediate forms uniting the two species. It is also possible that a comparison of a larger number of specimens may prove that the forms included under P. Thetis, P. symmetricum and P. bucculentum are varieties of one species.

Formations and localities. In the Upper Helderberg limestone at Darien, and in the Hamilton group at Darien, York, and Canandaigua lake, N. Y.

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PLATYCERAS BUCCULENTUM.

PLATE III, FIGS. 7, 26-29.

Platyceras bucculentum, HALL. Descriptions of New Species of Fossils, etc., p. 5. 1861.
Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 33. 1862.
Illustrations of Devonian Fossils: Gasteropoda, pl. 3. 1876.

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SHELL ventricose, obliquely subovoid. Apex extremely attenuate, the spire making one or two closely enrolled volutions, with a gently enlarging diameter, and below this abruptly expanding and becoming very ventricose in the middle and lower part; spreading more upon the right side than upon the left; the shell near the posterior side swells out into a distinct pouch-like projection, with two or three rounded folds or semiplications, which give a deeply sinuous outline to the margin. Aperture subovate, and sinuate on the right posterior side. Peristome sinuous, and on the posterior side spreading partially over the preceding volution. SURFACE marked by fine closely arranged concentric striæ, which are undulated towards the margin of the aperture, and sometimes over the greater part of the surface, the irregularity having commenced during the earlier stages of growth. In well-preserved specimens there are revolving striæ or fascicles, rising in little bands of obsolescent striæ, giving a waved aspect to the surface.

This species is more ventricose than any of the preceding forms. The character of apex, and the widely expanded body-volution resemble in some degree

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