CEP HA LOP DA. 435 



their .diameter, and gradually more in proportion as the outer volution 

 becomes expanded toward the periphery, embracing about two-thirds the 

 width of tbe preceding volution. Umbilicus large and open, exposing all the 

 volutions of the spire; its sides in the young shells rounded, and in the older 

 shells abrupt, leaving the margin obtusely angular, and descending almost 

 rectangularly into the cavity. A transverse section presents the inner 

 volution with a circular outline, the dorsal side of which is concave from 

 embracing the preceding volution; the dorso- ventral diameter is about three- 

 fourths as great as the transverse, varying in proportion at each successive 

 volution. In the outer volutions the section presents an elongate, trapezoidal 

 form, in which the dorso-ventral diameter is equal to once and a half to twice 

 the basal diameter, which is measured from the greatest expansion at the 

 margin of the umbilicus. The exterior or peripheral side is flattened in old 

 specimens and measures about one-half, or less than one-half, the diameter 

 across the base. The basal line is convex on each side, and concave in the 

 centre for the reception of the embraced volution; the lateral faces are limited 

 by gently curving lines, which become more nearly straight toward the 

 aperture. The increase of width is nearly or quite double for each volution. 

 Initial extremity small, globular, and closely surrounded by the succeeding 

 volutions. 



Chamber of habitation extremely large, occupying fully two-thirds of the 

 last volution, with capacity at least four times as great as all the air-chambers 

 together. Aperture of the same form as a section of the last volution, and, 

 judging from the best preserved specimens, it is slightly expanded on the 

 lateral margins, and much expanded at the baso-lateral angles, becoming 

 auriculate ; the anterior margin is deeply sinuate, with a shallower sinus in 

 the lateral margins. In imperfect specimens these features are indicated 

 by the striae upon the surface. 



Septa deeply concave, and with some exceptions, regularly increasing in 

 distance toward the outer chamber ; in mature specimens they are ten or 

 twelve mm. distant from each other, and in the extremely large specimens 

 more than fifteen mm. distant; sometimes several of the later septa are nearer 



