800 THE PALEONTOLOGY OF MINNESOTA. 
LOncoceras 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 ventralside 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 and 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. 
