the P. ventricosum of the Lower Helderberg group; but the first volution is more slender, and the spire less closely enrolled, while the lateral pouch-like expansion and the revolving bands are distinctive features. This shell is also much smaller, being rarely more than an inch and a quarter high, with the greatest diameter of the aperture about equal to the height. The length from the apex to the anterior margin of the aperture is about one inch. and a quarter in large individuals. The small specimen (fig. 7) is a very symmetrical form of a young individual of the species which preserves the peristome in contact with the preceding volution. The figure scarcely represents the true character of the specimen. Formation and localities. In shales of the Hamilton group, at York and Moscow, in Livingston county, N. Y. PLATYCERAS FORNICATUM. PLATE IV, FIGS. 1-5, 7, 8, 18, 19, 20, AND PLATE V, FIGS. 8, 9 [?]. Platyceras fornicatum, HALL. Descriptions of New Species of Fossils, etc., p. 7. 1861. 66 (?) 66 66 v. contractum, HALL. Id., pl. 5, figs. 8, 9. 1876. rictum (part), HALL. Id., pl. 4, figs. 18, 19, 20. 1876. SHELL obliquely subhemispherical, or very depressed, obliquely subconical. Apex minute, distinct, spirally enrolled for about one turn and a half, below which it expands, so that in the extent of an inch and a half along the dorsum to the front, it has acquired an aperture of about an inch and a half in diameter in both directions. The upper side of the spire for the first volution and the following half is flattened; the angle continuing into the broad expansion of the body-whorl, and dying out before reaching the margin, as shown in figs. 1-5. Aperture nearly round or roundovate; peristome scarcely sinuous, except at the postero-lateral margin. SURFACE marked by fine concentric striæ, with a few strong spines upon the body-volution. This species is conspicuously different from the P. dumosum in the shorter and comparatively equilateral and more expanded form; while the few scattered tubular spines appear to be disposed without order on the surface. A variety (figs. 8 and 9 of pl. 5), which may belong to this species, has a less expansion of aperture, an obliquely ovoid form, less attenuate apex, and greater proportionate height. Formation and localities. In limestone of the Upper Helderberg group, at Williamsville, and near Carlisle, N. Y. The varieties are from Darien, N. Y. PLATYCERAS CYMBIUM. 66 66 PLATE IV, FIGS. 9-11. Platyceras cymbium, HALL. Descriptions of New Species of Fossils, etc., p. 7. 1861. SHELL Subangularly ovoid. Apex minute, making one or more turns, below which the shell is abruptly expanded; the back flattened or but moderately convex, while the right side is distinctly flattened, making an obtuse angular ridge between the side and back of the shell: sloping more gently on the left side and becoming regularly convex. Aperture widely expanded. SURFACE marked by transverse striæ, which are undulated on the dorso-lateral angle. Shell lamellose striate. This species is characterised by the broad expansion of the aperture, the usually flattened dorsum, and the abrupt right or upper side, giving an obtuse dorso-lateral angle.. A careful examination of specimens renders it quite doubtful whether any of those with scattered spines really belong to this species. Formation and locality. In Upper Helderberg limestone, at Clarence Hollow, N. Y. PLATYCERAS RICTUM. PLATE IV, FIGS. 6, 12-17. Platyceras rictum, HALL. Descriptions of New Species of Fossils, etc., page 7. 1861. SHELL very depressed, arcuate, or sometimes obliquely subconical, the width equalling and often exceeding the height. Nucleus minute, the spire enrolled to about one turn and a half, when it abruptly expands, spreading more upon the right posterior side and becoming greatly extended and expanded in front. The dorsal line of the first volution is continued in an oblique angular ridge extending on the left side, and often reaching to the front of the shell giving a peculiar obliquity to the form. Aperture expanded, obliquely ovate; peristome entire, or with a slight sinuosity on the left posterior side, and sometimes showing the effect of the undefined folds of the posterior margin usually contiguous to the preceding volution. SURFACE marked by undulating concentric striæ, and a few broad undulations longitudinally; width of aperture from one inch to one and three-fourths, and a little greater from the anterior to the posterior margin; height about one inch and varying from three-fourths of an inch to an inch and a quarter. Formations and localities. In the Upper Helderberg limestone of Onondaga county, and in the shales of the Hamilton group in Ontario county, N. Y. PLATYCERAS ECHINATUM. PLATE V, FIGS. 1-4. Platyceras echinatum, HALL. Descriptions of New Species of Fossils, etc., p. 10. 1861. SHELL small, obliquely subovoid. Apex closely incurved for about one volution or one and a half; the body-whorl, from one-half to one volution, is ventricose, rapidly expanding from the first, giving an obliquely conical form. Aperture nearly circular or broad oval; peristome sinuate, the lines of growth and fine striæ conforming in direction to the outline of the margin. Remains of revolving striæ are sometimes traceable, when the shell is not exfoliated. Besides the concentric and less conspicuous revolving striæ, the surface is studded with numerous nodes or small spines the latter preserved only when the shell has been imbedded in soft shale, and quite separable from the rock; when imbedded in limestone, the spines and exterior shell are removed with the matrix. In this species the shell varies from half an inch to an inch and a quarter in length, and in a large specimen, the greatest diameter of aperture is one inch. In this form we have a miniature representation of the P. dumosum, and it might perhaps be considered as the young of that species if occurring in the same formation. Since, however, it is known only in a higher geological horizon, it must be regarded as a distinct species or a degenerate condition of P. dumosum. Formation and localities. In shales of the Hamilton group; Moscow and Ludlowville; and in Tully limestone, at Ovid, N. Y. PLATYCERAS DUMOSUM. PLATE V, FIGS. 11-16, AND PLATE VI, FIG. 1. Platyceras dumosum, CONRAD. Third Ann. Rep. Pal. Dept. [N. Y.] Surv., p. 205. 1840. 66 66 66 66 66 66 66 Compare Platyceras multispinosum, MEEK. Proc. Acad. Nat. Sci., Philadelphia, p. 17. 1871. 66 66 66 66 Geolog. Surv. Ohio: Pal., vol. 1, p. 210, pl 20, f. 7. 1873. SHELL Subovoid, arcuate, extremely ventricose when full-grown; the length from the apex to the anterior margin of the aperture greater than the height. Apex minute, closely enrolled for a single volution or more, when the body-volution becomes free and rapidly expanded, spreading more upon the right side, which is sometimes depressed-convex, while the left side is more abruptly rounded. The aperture is subrhomboid-ovate, with the peristome making a sinus on the left side, the posterior margin widely separated from the preceding volution. SURFACE marked by strong concentric striæ, which are interrupted and irregular from the numerous nodes projecting from the shell, and extended into long tubular spines. Formation and localities. From the Upper Helderberg group, Helderberg mountains, in Albany and Schoharie counties, N. Y.; Columbus, Ohio, and Falls of the Ohio river. This shell, in its full size, attains a length of two and a half inches, with the entire surface covered with strong spines which are sometimes two inches in length. In well-marked specimens, these spines are arranged in diagonal rows across the body of the shell in a nearly regular quincunx order. One specimen shows the bases of ninety of these spines, and, from the extent of the shell, the whole number must have been more than one hundred. Mr. CONRAD's description is as follows: "Shell covered with thick tubular spines, arranged in longitudinal rows; margin of aperture waved; volutions free." He remarks that, "in some varieties the spines are comparatively few." In some specimens of about one-half the full size, there are not more than fifteen or twenty spines; and in all those with few spines, the expansion of the body-whorl is much less abrupt, and they rarely attain more than half the size of the large typical forms of the species. The number of volutions in the smaller forms is fully equal to the larger ones, being two or more, with the last one quite free. Since this form is so distinct and constant, I propose to designate it as variety rarispinum. The Platyceras multispinosum of Meek, figured in the Geological Report of Ohio, presents the form and proportions of well preserved specimens of P. dumosum as they occur in the limestone of New York, but is somewhat larger than any specimens we have seen. It is described as "much |