4 PROCEEDINGS OF THE NATIONAL MUSEUM vol.76 



Typically each whorl overlaps the preceding to fully two-thirds its 

 height. By secondary deposits of lime the amount of overlap is 

 made even greater in the older portion of the shell. In section the 

 whorl is an asymmetrically compressed oval with the long axis at an 

 angle of about 45° to the axis of the shell. The inner surface, next 

 the umbilicus, is nearly straight and vertical. At the upper inner 

 shoulder a fairly sharp angular keel is formed. The outline of the 

 whorl in section is well shown in the illustrations. 



The surface of the shell when well preserved is marked by fine 

 rounded closely spaced growth lines. The course of these lines 

 can not well be followed in a single specimen, but by using several 

 individuals and a carefully prepared aperture in one specimen it 

 can be worked out in detail. The apertural margin that is figured 

 here was prepared by grinding away the matrix without cutting into 

 the shell. Some of the margin may have been broken away before 

 fossilization, but the aperture as illustrated is, I believe, essentially 

 the true one. It has been carefully checked by other specimens 

 that show the margin in part. Starting at the lower suture the 

 growth line on an adult whorl trends slightly backward and then 

 carries forward in a smooth curve to the periphery of the whorl. 

 Here a forward projected saddle is outlined. The growth line now 

 swings backward in a sweeping curve, which becomes less accentu- 

 ated in the upper fifth of the whorl as it approaches the angular 

 keel. Passing over the keel the line carries forward a little from 

 the vertical to the inner suture. This gives us a very deep asym- 

 metrical apertural notch with the apex at the keel. These structures 

 may clearly be seen in the illustrations. 



The central tubular cavity as noted above extends to the apex 

 of the spire. Each whorl is stepped outward slightly in relation to 

 the one preceding, giving an ever-widening cavity with advancing 

 age. In an adult individual, the outside measurements of which 

 have been given above, the umbilicus has a width in excess of 25 

 millimeters. 



An interesting feature shown by vortical sections is that the 

 whorls were progressively filled with secondary deposits of lime 

 during the life of the animal. In the section figured it will be noted 

 that only the two latest whorls show an inner cavity, the size of 

 which decreases rapidly as one goes backward. The older whorls are 

 solidly filled. Where there is still an opening in the whorl tbp 

 margin of the cavity shows as a smooth sharply defined wall. 



Horizon and localities. — Mitrospira longwelli where found and 

 the stratigraphic relationships known occurs m the lower portion of 

 the Chazyan part of the Pogonip limestone. As noted above, it 



