Air-chambers regular, numerous, gradually increasing in depth toward the grand chamber, varying from three to seven mm. in the length of twentyeight air-chambers. Septa smooth, moderately concave, with an areola around the insertion of the siphuncle. Sutures straight and at right angles to the spiral axis.

Siphuncle near the ventral side, moniliform; not satisfactorily observed. Test and surface-markings unknown. Internal mould essentially smooth, with the sutures moderately impressed.

The specimen figured on pl. 100 has a diameter, measured from the aperture across the disc, of 180 mm., and the greatest diameter of the tube is fifty-two mm.

This species is distinguished from G. spinosum by the absence of revolving rows of spines or nodes, and a more gradually enlarging tube. It differs from G. Eryr in its smaller volutions and more openly coiled spiral.

Formation and localities. In the Schoharie grit, Schoharie and Albany counties, N. Y.

GYROCERAS ERYX.

PLATES LVIII, FIG. 1; CIII, FIG. 3.

Gyroceras Eryx, HALL. Descriptions of New Species of Fossils, etc., p. 39. 1861.

66

66

66

66

66

64

Fifteenth Rep. N Y. State Cab. Nat. Hist., p. 67. 1862.

Illustrations of Devonian Fossils: Cephalopoda, pl. 48, fig. 1. 1876.

SHELL large, discoid, coiled, making about two nearly contiguous volutions. Transverse section broadly oval; ventro-dorsal diameter the longer. Tube regularly enlarging from the apex to the aperture. Apical angle about 10°. Chamber of habitation large. Aperture not observed.

Air-chambers numerous, regular, having, on the outer volution, a depth of fifteen mm., measured on the convexo-ventral side, and of six mm. on the concavo-dorsal side. Sutures straight and at right angles to the spiral axis

of the tube.

Siphuncle ventral, near the chamber walls, nummuloid, expanding to a diameter equal to the depth of the air-chambers.

Test and surface-markings unknown. Internal mould smooth, with the sutures slightly impressed, and preserving some traces of transverse lines of growth.

The largest individual observed has a diameter, measured across the disc, of 200 mm., with a diameter of the tube at the aperture of seventy mm. A small, septate fragment, comprising about one-half of one volution, has a diameter across the volutions of 110 mm., and preserves about thirty airchambers.

This species is distinguished by its transverse section, its size, closely coiled volutions, and the absence of any prominent ornamentation on the tube. It somewhat resembles G. validum in its smooth internal mould, but the large, closely coiled volutions and transverse section are very different. From G. Cyclops it is easily distinguished by the absence of the annulations and crenulations of the internal mould, and is without evidences of any characteristic surface ornamentation.

Formation and locality. In the Magnesian limestone of the age of the Hamilton group, near Milwaukee, Wis.

GYROCERAS CYCLOPS.

PLATES CI; CII; CIII, FIGS. 1, 2; CIV, FIGS. 1, 2.

Gyroceras Cyclops, HALL. Descriptions of New Species of Fossils, etc., p. 40. 1861.
Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 68. 1862.

66

66

66

Illustrations of Devonian Fossils: Cephalopoda, pl. 53, figs. 1-3. 1876.

SHELL large, discoidal, regularly coiled. Spiral open, making about one volution and a half. The volutions are distant about twenty mm.; near the aperture they are almost contiguous on account of the expansion of the tube. Transverse section subcircular, or broadly oval, flattened on the dorsum, and obtusely subangular on the ventral side. Tube regularly enlarging from the apex. Apical angle about 14o.

Chamber of habitation small, expanding and forming a campanulate aperture, which opens outward, oblique to the spiral axis of the tube. This

feature of the direction of the aperture is inferred from the lines of grow th and the direction of the ornamentation of the test. The crenulations are preserved as fine, rounded ridges and striæ, along the internal mould of the walls of the air-chambers.

Air-chambers frequent, regular, becoming numerous toward the apex, having a depth of ten mm. near the grand chamber, and of one mm. near the apex. Septa smooth, with a very slight concavity, equal to an are of about 60°. Sutures straight, and at right angles to the spiral axis.

Siphuncle near the ventral side, small in its passage through the septa; having a diameter of six mm. where the tube has a ventro-dorsal diameter of forty-five mm.

The test has a thickness of about one mm. on the outer volution. Surface marked by irregular, lamellose, undulating lines of growth, which slope in a retral direction, oblique to the spiral axis of the tube. The ornamentation consists of strong, transverse, prominent, plicate, foliate expansions of the test, inclining toward the aperture, and having an elevation of about twentyseven mm. on the outer volution, growing less prominent and more finely plicated toward the apex. These expansions are quite regularly plicate, and present surface-markings similar to the general surface of the tube. The folds or plications are continued along the tube, forming rounded, revolving ridges. The sinus in the striæ and ornaments of the test is rounded, and has a width equal to twice the depth.

The internal mould is annulated from the strong transverse expansions of the test, and shows the furrows and ridges of the crenulations, with those of the test. Sutures not impressed.

One specimen, nearly entire, constituting about one volution and a half, has a length of 600 mm., with a diameter of seventy-five mm. near the aperture, and measures 210 mm. from the ventral margin of the aperture across the volutions.

This species is distinguished by its size, the curvature of the tube, the apical angle, and the prominence of the ornamentation. The apical portion differs from G. trivolve in its more rapidly enlarging tube and marked characters of the

internal mould and surface ornaments. In G. Nereus the tube is more slender, less involute, and the expansions of the test are more frequent and more decidedly plicated, but much less elevated. The curvature of the tube in G. Matheri is distinctive, and the tube is more slender, and with a flattened, transverse section. This form is a remarkably large and well-defined species, and has a horizontal range greater than any of the previously described forms, except, perhaps, C. amulum.

Formation and localities. In the limestones of the Upper Helderberg group, Helderberg mountains, N. Y.; near Columbus, O.; and Kelly's Island, Lake Erie.

In the historical notices and descriptions of species of GYROCERAS and CYRTOCERAS, no mention has been made of the numerous genera and subgenera which have been constituted on various modifications of the typical forms of these genera. The majority of the species of CYRTOCERAS and allied genera have been indicated from the Silurian system. And on the other hand, the greater proportion of the species of GYROCERAS have been described from Devonian and Carboniferous strata. From this fact, it would seem probable that there was sufficient grounds for some generic separation, and that these genera held an equivalent relative position in these grand divisions of paleozoic time.

In the Silurian system the genera ORTHOCERAS and CYRTOCERAS form the leading feature in the cephalopodous fauna, while in the succeeding periods the genus NAUTILUS supersedes ORTHOCERAS, and GYROCERAS appears to be a gradual and natural outgrowth from CYRTOCERAs.

TROCHOCERAS, BARRANDE-HALL.

The genus TROCHOCERAS was preresed by M. BARRANDE and by myself, about the same time, and quite independently of each other, for fossils of identical character and structure. The name was first The name was first published by M. BARRANDEmy own appearing only in the second volume of the Paleontology of N. Y., Recognizing these facts, M. BARRANDE has done me the honor to associate my name with his own in his subsequent publications of the genus, and I have adopted the same course.

In 1852 J. HALL described Trochoceras Gebhardi and T. turbinatum, from the Coralline limestone of the Niagara group (Palæontology of New York, vol. 2).

In 1860 Mr. J. H. McCHESNEY described T. Desplainense, from the Niagara group (New Paleozoic Fossils).

In 1861 J. HALL described T. Clio and T. eugenium, from the Schoharie grit (Fourteenth Rep. N. Y. State Cab. Nat. Hist.); T. costatum, from the Niagara group of Wisconsin (Geolog. Survey of Wis.: Rep. of Progress); and T. discoideum, from the Schoharie grit (Desc. New Species Foss.).

In 1865 Messrs. MEEK and WORTHEN described T. Bari [=Nautilus], from the Ideon-river group (Proc. Acad. Nat. Sci. Phila.).

Meeers. WINCHELL and MARCY described Gyroceras Bannisteri [=Trochoceras], from the Niagara group of Illinois.

In 1867 J. HALL described T. notum, from the Niagara group of Illinois Teeth Rep. N. Y. State Cab. Nat. Ilist.).

1. 1869 M. BARRANDE described T. incipiens, from the Quebec group (Syst. A centre de la Boheme).

121870 J. HALL described T. Eneas, from the Niagara group of Iowa verseth Rep. N. Y. State Cab. Nat. Hist.: Revised ed., explanation of pl. 25). 11876 J. HALL illustrated T obliquatum, T. Pandion, Cyrtoceras Orion [=T. Good, from the Schoharie grit (Illustrations of Devonian Fossils); and T. Walsense, from the Niagara group of Indiana (Twenty-eighth Rep. N. Y. State Mus. Na. Hist.).