CEPHALOPODA. 441 



GONIATITES DISCOIDEUS. 



FLAXES LXXI, FIGS. 1-13; LXX1V, FIGS. 4, 5. 



GoniaVtex dUfokleiis, Hall. Thirteenth Rep. N. Y. State Cab. Nat. Hisl., pp. 97, 98. figs. 3-6. 1860. 

 " u Twenty-seventh Rep. N. Y. State Mus. Nat. Hist., p. 136. 1875. 



" " " Illustrations of Devonian Fossils : Cephalopoda, pi. 71, 1S76. 



" stnuosus. Hall (in part). Geolog-. Surv. N. Y. : Rep.. Fourth Dist., p. 246. 1843. 



Shell depressed orbicular in the young state, becoming discoid in its advanc- 

 ing growth and varying conditions of preservation. In young shells the 

 transverse and lateral diameters are about as five to ten, eleven to twenty, 

 and twelve to twenty-four ; in a medium sized specimen, preserving its 

 natural proportions, the two diameters are as seventeen to forty-six A speci- 

 men, which is perhaps somewhat compressed, has a lateral diameter of sixty 

 mm., with a transverse diameter of twenty mm. The lateral faces curve 

 rapidly in young shells, and in older ones are very gently curving toward 

 the periphery, which is abruptly rounded. " 



Volutions about four, embraced within the outer one, having the umbilicus 

 closed exteriorly, and with a slFght depression, which does not expose the 

 inner whorls ; somewhat gradually expanding in the young shell and more 

 rapidly in the older ones. Transverse section, semi-elliptical with the base, 

 deeply concave from the inclosed preceding volution. The increase in width 

 of the last volution, in young, well-formed shells, is from eight to fifteen mm. 

 In a large form it is from fourteen to twenty-eight mm., and in a specimen 

 of sixty-five mm. in its greatest diameter, the increase in the last volution is 

 from twenty-five to forty mm., while the largest specimen measured, which 

 is much compressed and somewhat distorted, gives thirty-two and sixty mm. 

 as the diameters of the base of the last volution and that at the aperture. 



Chamber of habitation very large, having an area four times as great, and 

 a capacity eight or ten times as great as all the air-chambers. Aperture, in 

 old shells, elongate-semielliptical, with the base deeply indented by the 

 embraced volution ; toward the base the margin is laterally expanded, and 

 distinctly auriculated at the baso-lateral angles; thence curving rapidly 

 forward, it is again depressed anterior to the middle of the lateral face, 

 forming a shallow sinus ; whence it advances on the ventro-lateral portions, 

 56 



