474 P A LMO .V TOLOO Y OF NEW YORK. 



GONIATITES IXION. 

 PLATES LXXIII, FIUS. 14-U ; I.XXIV, FIG. 12. 



<t„niatiU* rotatoriusf db Koxixck. Hall : Thirteenth Hep. N. Y. State Cab. Nat. Hist., p. 101, figs. 15, 16. 

 . Hall. Thirteenth Rep. N. Y. State Cab. Nat. Hist., p. 125, tigs. 1-3. 1S60. 

 «• « " Illustrations of Devonian Fossils: Cephalopoda, ]>1. 73, figs. 12-14. 1876. 



ix discoid, with the transverse and lateral diameters nearly as one to three ; 

 greatest thickness at a point distant from the umbilicus about one-fourth of 

 the diameter of the volution, and the surface sloping thence with a gentle 

 curve to the peripheral margin. The younger specimens are more rotund, 

 having a thickness equal to half the greatest lateral diameter. 



Volutions seven or eight; their lateral diameter equal to about once and a 

 half the transverse diameter. Umbilicus closed, or very small in the casts of 

 the interior, and the inner volutions not at all exposed. Transverse section 

 semi-elliptical, with the height equal to about once and a half the width at 

 the base ; the sides gently curving and converging to the peripheral margin, 

 which is somewhat abruptly rounded ; base indented to nearly half its height 

 by the preceding volution. The rate of enlargement of the volutions in a 

 specimen having a greatest diameter of eighty mm. is from twenty-eight to 

 fifty-two mm. — an increase of twenty-four mm. in a single volution. In a 

 small specimen, measuring eighteen mm. in its greatest diameter, the outer 

 volution increases from six to thirteen mm., or seven mm., in a single turn. 

 In a specimen of forty-five mm. in its greatest diameter, the last volution 

 enlarges from fourteen to twenty-six mm., or an increase of twelve mm. 



Chamber of habitation unknown. Aperture not observed, but probably 

 having the same form as the transverse section. Air-chambers numerous, 

 regular, shallow ; their greatest depth on the middle of the lateral faces of 

 large individuals is five, and rarely six mm., and the depth at the peripheral 

 margin about twelve mm., while at the umbilical margin the depth does not 

 exceed one mm. In the smallest specimens observed, the depth of the cham- 

 bers on the lateral face is less than two mm., while on the peripheral margin 

 the depth is about five mm. 



