144 BOOK OF SPECIES. 
POLYSTOMELLA. 
Shell discoid, multilocular ; whorls contiguous, not 
apparent externally, radiated exteriorly by strie 
or coste running in the direction of the whorls; 
aperture of many holes variously disposed. 
P. crispus. W. 18. 8. 
VORTICIALIS. 
Shell discoidal, spiral, multilocular; whorls con- 
tiguous, not apparent externally ; septa trans- 
verse, imperforate, not extending from the 
centre to the periphery; aperture marginal. 
V.sTRIGILATA. W. 13. 41. 
NAUTILUS. 
Discoidal, spiral, multilocular ; parietes simple, 
without any suture; whorls contiguous, the 
last enveloping the sutures; chambers numerous, 
narrow, transverse, formed by transverse septa, 
last chamber very large; septa concave on the 
side next the aperture, thew discs perforated 
by a tube, and their margins very simple. 
N. pompiuius. Sub-orbicular, with the whorls 
smooth both on the back and sides; aperture 
oblong, heart-shaped; umbilicus concealed; white, 
