37 



GONIATITES ELKHORNENSIS, U. sp. 



Plate IV, Fig. 9, lateral view; Fig. 10, ventral view; Fig. 11, 

 surface form of a septum. 



Shell very large, discoid. There are between three and four 

 volutions preserved, in our specimen, and, apparently, an entire 

 individual would have seven or eight volutions, all of which are 

 exposed, in the very wide shallow umbilical cavity or depression. 

 The volutions are rolled, in the same plane, and increase more 

 rapidly, in transverse, than in the dorso-ventral diameter. At first 

 the transverse diameter is not greater than the dorso-ventral, but 

 later, as show a by the ventral view in figure 10, the transverse 

 diameter becomes more than twice as great as the dorso-ventral. 

 The ventral side is slightly convex and the dorsal side correspond- 

 ingly concave, which allows the volutions to be very closely coiled, 

 without properly overlapping. The inner volutions are beveled, 

 on the lateral sides, from the middle part, leaving a middle an- 

 gular ridge, which gradually approaches, in the last volutions, the 

 ventral margin, and, at the body chamber, forms an angle, at the 

 ventral margin, from which the beveled edge extends to the next 

 inner volution. 



The external shell of our specimen is not preserved. The air 

 chambers are very long, but do not increase, in length, in propor- 

 tion to the increasing size of Ihe volutions, but, on the contrary, 

 do not seem to increase in length at all. Some of the septa are 

 not distinct toward the end of the last volution, and it is not 

 clear whether or not any part of the body chamber is preserved, 

 but the ventral side indicates that another volution is necessary 

 to complete the shell. The septa are distant. They curve gently 

 backward from the umbilicus and then forward each one crossing 

 the lateral side of a volution in a sigmoid flexure, and are then 

 more abruptly directed backward over about one-third the width 

 ot the ventral margin, where each one is abruptly bent forward 

 and forms a semielliptical arch across the middle of the ventral 

 side. There are, therefore, three convex saddles, the middle one 

 being semi-elliptical, and extending only about half as far forward 

 as the less convex lateral saddles do. This will be better under- 

 stood by looking at the ventral view and the surface form of a 

 septum as represented in the illustrations. 



This species is distinguished by the general form of the shell, 

 manner of enrollment of the volutions, transverse section of a 

 volution and by the saddles and lobes in the septa. 



