CEPHALOPODA. 433 



GoNIATITES MlTIIRAX. 



PLATES I.XIX, FIG. 7; LXXIV, FIG. 14. 



Qoniatites Mithrax, Hall. Thirteenth Rep. N. Y. State Cab. Nat. Hist., pi. 9S, fig-. 7. 18C0. 



" " Illustrations of Devonian Fossils : Cephalopoda, pi. 69, tig. 7. 187C. 



Shell suborbicular, 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 periphery, 

 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 

 55 



