800 THE PALEONTOLOGY OF MINNESOTA. 



lOncooeras lycus. 



broader curvature on the dorsal margin ; expanding rapidly to its greatest 

 diameter near or at the base of the body-chamber. About the aperture it is 

 broadly and rather deeply constricted and the margin slightly expanded and 

 reflected. This apertural contraction is greatest on the ventral side where it is 

 pinched out to a very narrow, subacute angle and the outline thus given to the 

 aperture is that of an acute oval. The transverse diameter of the aperture is less 

 than that of the body-chamber below the constriction. Its lateral margins form 

 low convex curves bending into slight concavities on the dorsal and ventral sides. 

 Transverse section of the septate portion of the shell broadly oval, the dorso- 

 ventral and lateral diameters being about as 9 to 10. The greatest breadth of the 

 section is somewhat within or toward the dorsal side of the center of the septum. 

 Thus the surface of the shell-tube, which is quite broad on the dorsal curve, in- 

 creases in breadth for about one-third of its lateral extent, thence decidedly 

 narrows to the venter which is subacute. The septa are gently and regularly 

 convex over the earlier part of the shell but increase in this respect near the body- 

 chamber. The greatest depth of the septa is within the center. The sutures make 

 broad and low, scarcely perceptible lateral lobes and dorsal saddle, but their 

 anterior curvature is decidedly marked upon the venter. The depth of the air- 

 chambers varies somewhat in different specimens and in the same individual, those 

 near the body-chamber being as a rule the shallower. In a distance equal to the 

 greatest diameter of the shell there are eight air-chambers in one example, and 

 nine in another. In all of these the depth on the dorsal curve is scarcely more 

 than one-half that on the venter. Sipho ventral, situated just within the margin, 

 somewhat expanded between the septa. Some of the internal casts indicate that 

 the external surface was smooth arid covered with concentric growth lines which 

 were strongly reflected over the venter in a direction just the reverse of that of 

 the septa. Some of the casts bear very obscure traces of longitudinal ridges which 

 may be altogether of muscular origin. 



Dimensions. A well preserved example lacking a portion of the apex, measures 

 60 mm. on the ventral curve and 31 mm. on the dorsal curve. The body-chamber 

 is 21 mm. in length and lacks a portion near the aperture. In 35 mm. on the ventral 

 curve there are twelve air-chambers. The transverse lateral diameter of the 

 shell where thickest is 19 mm. and the dorso-ventral diameter 24 mm. In another 

 specimen which has a dorso-ventral diameter of 18 mm. at the third septum and 

 retains the body-chamber and aperture in their entirety, the length of the body- 

 chamber is 20 mm.; of this, 11 mm. occur below the constriction. The major and 

 minor diameters of the body-chamber at its base are 24 and 20 mm., the major axis 

 of the aperture is 23 mm. and its greatest lateral axis 15 mm. 



