CEPHALOPODA. 
433 
Goniatites Mithrax. 
PLATES LXIX, FIG. 7 ; LXXIY, FIG. U. 
Goniatites Mithrax , Hall. Thirteenth Rep. N. Y. State Cab. Nat. Hist., pi. 9S, fig. 7. 18G0. 
“ “ “ Illustrations of Devonian Eossils: Cephalopoda, pi. 69, fig. 7. 1876. 
Shell suborbioular, discoid or very depressed, the thickness of the disc being 
about one-fourth of its diameter; acutely rounded on the periphery. Volu¬ 
tions probably four or more; the number not determined. Outer volution 
embracing the inner ones for their entire extent, leaving the umbilicus closed. 
The increase in width of the outer volution, as preserved, is from 3.5 to 5. 
Transverse section of the volution elongate, semi-elliptical, the dorso-ventral 
and transverse diameters being about as two to one. Base truncate at the 
umbilicus, and deeply indented by the preceding volution; apex acutely 
rounded; lateral faces gently curved to the middle of the deep lateral lobe, 
and thence a little more rapidly to the periphery. 
Chamber of habitation unknown. Aperture unknown, but inferring from 
the form of the volutions, it has been semi-elliptical, with the base deeply 
concave. Air-chambers regularly increasing in depth with the growth of 
the shell, the increase in a single volution being from eleven to fifteen mm. 
(=four mm.), as measured on the lateral lobe; about five to seven mm. a 
little.within the middle of the lateral face, and from thirteen to more than 
twenty mm., measured on the periphery. 
The septa curve gently forward from the umbilicus for nearly two-thirds 
of the width of the volution; thence more abruptly backward, forming a 
broad low undefined saddle, to a point nearly three-fourths of the width of 
the volution, when they again bend forward to the margin of the jieriphery, 
leaving a broad, deep lobe, which occupies nearly one-third the width of the 
volution; and thence turning abruptly backward to near the centre of the 
periphery, and sharply recurving, leave an acute triangular' saddle on each 
of the margins, and a narrow, acute, ventral lobe. The saddle occupying the 
centre of the short, ventro-lateral curve is acute at the summit, having a 
height one-fourth greater than the width at the base, and curving a little 
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