This species differs from all the other forms with which we are acquainted. Although bearing some general resemblance to Goniatites Oweni, it differs in very important particulars, especially in the closed umbilicus, the less rotund periphery, and the simple, linguiform, mucronate, ventral lobe. The species was originally identified with G. rotatorius, DE KONINCK, to which it bears a very close resemblance. An examination of the original of DE KONINCK's species shows a greater rotundity of the periphery of the volutions, and a depressed area around the umbilicus, corresponding with the description given. These features may also be observed in a comparison of the figures of the two species. G. rotatorius is stated by DE KONINCK to have a diameter of eleven centimetres, which nearly corresponds with the figure given by him. In that figure nineteen air-chambers are shown; and in a specimen of G. Ixion, of eighty-five mm. in diameter, twenty-two air-chambers are exposed. A specimen of G. Irion, having its greatest diameter forty-seven mm., exposes nineteen air-chambers in its entire outer volution. Noting such differences in the character of the two forms, I am still disposed to continue the American form under a distinct designation. In the general form of the shell and the direction of the septa, this species may be compared with G. Mithrax, in which the lateral lobe is obtuse and rounded at its extremity. Formation and locality. In the Goniatite limestone of Rockford, Ind. Goniatites Lyoni, M.-W. Proc. Acad. Nat. Sci. Phila., vol. 12, p. 471. October, 1860. 46 66 Hyas, HALL. 46 Lyoni, M.-W. Geolog. Surv. Illinois, vol. 2, p. 165, pl. 14, fig. 11. 1866. SHELL discoid, depressed, often flattened from compression; the thickness of the disc is from eight to ten mm. where the lateral diameter is from sixty to eighty mm. Volutions six or more, in entire individuals, all exposed in the very wide, shallow umbilicus, a small proportion only (less than one-fifth) of the inner being overlapped by the outer ones, which are grooved upon their inner ѵѵѵ, шии a very паrrow and This species differs from all the other forms with which we are acquainted. · Although bearing some general resemblance to Goniatites Oweni, it differs in very important particulars, especially in the closed umbilicus, the less rotund Prolecanites Lyoni Halk Week and Worste Plate XVI Fry 18 1860 1864 9 Comatites you, theek and Worthen, Pase. Head, Nat. Sci. Phil d. Prolecanites marshallensis & inchell Comictites mais exhalinsin, A. Winchell, All Your Fai Volutions six or more, in entire individuals, all exposed in the very wide, shallow umbilicus, a small proportion only (less than one-fifth) of the inner being overlapped by the outer ones, which are grooved upon their inner margins for the reception of the periphery of the inclosed volution; the groove has a depth of about two mm. where the dorso-ventral diameter of the embracing volution is twenty-one mm. Transverse section of the outer volutions trapezoidal (of the inner ones semi-elliptical), the sides nearly flat, the apex truncated, with the angles rounded; the base is concave in the middle, with the baso-lateral margins rounded. The greatest transverse diameter is equal to two-thirds of the dorso-ventral diameter, the difference in the proportions increasing in the outer volutions. The volutions enlarge very gradually; the measurements give seven and thirteen, and eleven and eighteen, or an increase of six and seven mm. in a single turn. In a larger individual the increase in half a volution is from nineteen to twentythree mm., or equal to eight mm. in a single volution. . Chamber of habitation unknown, all the specimens observed being incomplete. Aperture unknown. Air-chambers numerous, slightly irregular in depth, and as a rule, gradually increasing toward the outer chamber. Septa strong, distinctly thickened and imbricating at their margins, and remarkable for the regularity of their curvature. From their origin they advance toward the aperture, in a nearly direct line, obliquely over the umbilical margin to a point on the lateral face about one-fifth of its diameter; thence bending abruptly backward they describe an extremely abrupt curve, including a linguiform mucronate lobe, and return to a point in the centre of the volution greatly in advance of their first retral bending. From the summit of this curve or saddle on the centre of the face of the volution they describe a somewhat longer and scarcely wider retral and advancing curve, which in like manner limits a narrow linguiform and mucronate lobe, and from which the septum curves over the margin of the periphery, thence abruptly descending and turning acutely forward describes a narrow triangular mucronate lobe on the ventral side. This order gives an obtusely subtriangular saddle on the umbilical margin, a narrow lobe, and a very narrow and elongate lateral saddle, the apex of which is almost precisely in the centre of the lateral face of the volution, and much in advance of the summit of the umbilical saddle; on the outside of this is another lobe similar to the first, and a peripheral saddle in which the summit is still more advanced than the one on the umbilical margin, and less advanced than the central one. These characters of the lateral face are better shown in figure 12 of plate 72 than in the figures on plate 73. The suture-lines are strongly marked and usually deeply impressed upon the weathered surfaces. Siphuncle minute; not satisfactorily determined. Test and surface-markings unknown. Upon the peripheral margins of some of the casts there is an impression indicative of the imprint of curving striæ, which may mark the margins of the sinus. The internal casts are smooth, with the exception of the deep indentation along the suture lines. The fossil usually occurs in a fragmentary condition, and few entire individuals have been observed. These measure from sixty to sixty-five mm. in their greatest diameter; but fragments of other individuals of larger size indicate that the shell has had a diameter of eighty or ninety mm., exclusive of the chamber of habitation. The slender and much expanded form has rendered it more easily broken and dismembered than the other species with which it is associated. This species is quite unlike any other known to me in our formations, though presenting, in its septa, some general resemblance to G. Chemungensis. Formation and locality. This species occurs in the same association with G. Ixion, in the Goniatite limestone, at Rockford, Indiana. |