The vertical distribution of the species of Orthoceratites, as indicated in the preceding table, offers some facts of general interest. The Potsdam sandstone, with its wide geographical range in the United States and Canada, has not afforded a single species of ORTHOCERAS, although prolific in its Trilobitic fauna, and its Linguloid and Oboloid forms, which are known in numerous localities over an extent of more than two thousand miles. In these respects the formation corresponds to the primordial zone of Europe. In the Calciferous, Quebec and Chazy groups we have fifty-three species of ORTHOCERAS and three of ENDOCERAS; of which, only three species pass into any succeeding formation. In the Black-river and Trenton groups we have sixtyone species of ORTHOCERAS and twenty-two of ENDOCERAS; of which, only four pass into succeeding formations. The Hudson-river group, including the Utica slate and Anticosti group, contains thirty-five recorded species of ORTHOCERAS and two of ENDOCERAS; of these, two species pass upward from the Trenton limestone. While there are the most intimate relations between the fauna of the Trenton and Hudson-river groups in all other classes, the Cephalopoda form a marked exception. In the Clinton and Niagara groups we have sixtytwo species of ORTHOCERAS recorded. In the Lower Helderberg group we have but nine species recorded, although a few other forms are known to occur in that horizon. This is in most striking contrast with the Niagara group, while the fauna in the other classes strikingly correspond. In the character and abundance of the Brachiopoda these two formations are almost identical; while the Gasteropoda are even more abundant in the Lower Helderberg than in the Niagara group. In the growth and development of the Corals and Bryozoa these formations are extremely similar; and there is a great similarity in the physical conditions of the groups throughout, except in the absence, to a great extent, of the deposition of Magnesian limestone in the Lower Helderberg period. In the Upper Helderberg group, of which the total thickness is less than the Lower Helderberg, we have thirty species of ORTHOCERAS recorded; while the Hamilton group, with a thickness more than five times as great, has yet afforded but twenty-nine species. The Chemung group, with its greater thick ness, but of a less calcareous character, has afforded nineteen species; while the Waverly, the Lower Carboniferous limestones and the Coal Measures have recorded seven, eight and eight species respectively. So far as we now perceive, the development of the Orthoceratites does not depend upon the duration of the deposition, or thickness of the formation. The Black-river limestone, of a few feet in thickness, and the Schoharie grit, of not more than thirty feet in thickness, have each as many species of ORTHOCERAS recorded as any single group, except the Hudson-river, Niagara, and the Hamilton. The following table shows the vertical distribution of eighteen species of the preceding table, which are known to occur in more than one geological formation: SPECIES. ORTHOCERAS, Breynins: hastatum, Billings laqueatum, Hall Minganense, Billings Murrayi, Billings planoconvexum, Hall Sieboldi, Billings tenuifilum, Hall virgulatum, Hall LOWER SILURIAN. MIDDLE SILURIAN. ORTHOCERATA OF THE SCHOHARIE GRIT. DESCRIPTIONS OF SPECIES. SECTION a.-Robust forms, showing a gradation from a simple areola on the septum surrounding the siphuncle, to a decided and complex organic deposit. ORTHOCERAS PELOPS. PLATES XXXV, FIGS. 1-3; XXXV A, FIGS. 1-6; XXXVII, FIGS. 3, 4; LXXVIII B, FIG. 2. Orthoceras Pelops, HALL. Descriptions of New Species of Fossils, etc., p. 45. 1861. 64 66 66 66 1862. in part. Illustrations of Devonian Fossils: Cephalopoda, pl. 35. 1876. SHELL robust, straight, elongate. Transverse section circular in specimens not compressed; the flattening often amounts to a change in the relations of the diameters of from 1-1 to 1-3. Cone gradually and regularly enlarging from the apex. Apical angle 7° to 910. Initial extremity unknown. Chamber of habitation well developed, sometimes measuring more than twenty-five centimetres in length, and at least five or six times as long as the diameter at the last septum; a broad, gentle constriction near the aperture is generally present. Depth of the air-chambers about one-sixth of the width of the tube; four to eight in the space of sixty mm., depending on the diameter of the tube or the distance from the apex. Septa thin, with a small areola around the insertion of the siphuncle, regularly concave; concavity equal to an arc of from 120° to 12610; part of this variation being probably due to compression, or other influences which have affected the true curvature. In the more perfect specimens, the sutures are horizontal and straight, the various degrees of distortion observed having in many cases made them curved and oblique to the axis. Siphuncle central, or very slightly excentric; walls thin, seldom preserved; width equal to about two-thirds the depth of the adjacent chambers, or approximately from four to five mm. The presence of an areola on the septa indicates that the siphuncle was moniliform, or slightly expanding. Test about one mm. thick, preserved in very few of the numerous individuals observed. Surface marked by irregular, lamellose striæ of growth. Some of the specimens referred to this species have been more than sixty centimetres in length. The diameter of the tube towards the aperture in the larger specimens is from sixty to one hundred mm. In all the specimens examined, the interior is filled with the same material as the surrounding rock. The septa are often partially destroyed, broken or distorted, and the siphuncle, as shown in figs. 2, 4, of plate 35 A, is very imperfectly preserved, showing no decisive characters in sections thus far made, or any evidence of an organic deposition. The specimen, fig. 1, pl. 35 A, has a length of thirty-five centimetres, and is imperfect at both extremities (a part only being figured). This species is scarcely known except in the condition of casts of the interior, of which specimens are rarely found having a length of sixty centimetres, but are always imperfect. In the ordinary conditions of preservation, where the specimens are more or less compressed, and imbedded in a coarse grit, it is impracticable to recognize the less conspicuous differences of character between this and other species, the most marked feature being the larger size and greater distance of the septa in this, as compared with other similar forms in the same rock, except O. masculum, in which the distance between the septa is so much greater as to be readily distinguished. From O. Zeus, this species is distinguished by its smaller apical angle and lesser convexity of the septa; the other characters are not unlike. This form bears some degree of external resemblance to a species in the Hamilton group (0. Eriense), but the internal characters are quite distinctive. In its vertical range it is confined, so far as known, to the Schoharie grit, the variety formerly designated as O. Ohioense, from the Upper Helderberg limestone of Ohio, proving a very distinct species. Severa! large, imperfect individuals have been observed among collections made from the Upper Helderberg limestone of Cherry Valley, N. Y., of Ohio, and Ontario, Canada, which possess characters in common with O. Pelops. The apical angle is about 7°. Air-chambers slightly irregular in depth, varying Siphuncle central. One specimen, preserving the chamber of habitation and twelve air-chambers, has a length of 230 mm., with a diameter of seventy mm. at the largest part of the tube. The septa are more frequent than in undoubted O. Pelops; but without more distinctive material, these forms from the limestone cannot be specifically characterized. from three to four in the space of thirty-five mm. Formation and localities. In the Schoharie grit at Schoharie, and at numerous localities in the Helderberg range, and in the outcrop of this formation along the Hudson River valley as far as Orange county, N. Y. It probably occurs in the same formation in New Jersey, as it appears to be coextensive with the Schoharie grit. ORTHOCERAS ZEUS, n. sp. PLATE LXXV, FIGS. 1-3. SHELL robust, forming a straight, elongate cylindro-conical tube, which diminishes very gradually in receding from the chamber of habitation. Transverse section circular. The apical angle is about 810, measuring from fragments preserving the chamber of habitation and the lower septate portion. Initial point unknown. Chamber of habitation large, and subcylindrical, being about twice as long (in an imperfect specimen) as the diameter at its base, and very gradually expanding towards the aperture, below which it is marked by a wide, shallow, curved constriction. Aperture unknown, but the tube is apparently gently expanded beyond the constriction. Air-chambers large. Septa thin, and distant from each other about one-fourth the diameter of the tube, or in actual distance, where the tube measures from fifty to sixty mm., they are about twelve mm. distant from each other. Surface of the interspaces, or casts of the interior of the chambers smooth, and flat on the exterior. Concavity of the septa equal to about eighteen mm., one-third the diameter of the tube, or equal to an arc of 132°. Siphuncle central, having a diameter of five mm. where it penetrates the |