212 FireLp CoLumMBIAN MusEUM—GEOLOGY, VOL. I. 
the uncovered nodes forming the inner of the two outer rows on the suc- 
ceeding volution within; umbilicus about equaling the greatest dorso- 
ventral diameter of the last turn. Young examples, half an inch to 
one inch in diameter, with coste linear, closely arranged, of nearly 
uniform size, and manifesting scarcely any tendency to develop nodes 
but already showing the forward curve of their outer ends well 
defined, while the peripheral keel is low, narrow and simple, and the 
furrow on each side shallow. At a somewhat larger size, coste 
usually more or less unequal in size, the larger ones now beginning 
to develop the two nodes at their outer curved ends, and to become 
a little more prominent and compressed at their inner extremities, 
while the rather more prominent keel begins to develop its crenate 
outline, and the nodes nearest to assume their compressed form and 
parallel arrangement. On attaining to two and one-half to three 
inches in diameter, costae, nodes and keel become more prominent, 
the latter being strongly compressed and deeply and largely scal- 
loped, with divisions rounded in outline; while at this stage of growth 
the periphery, as seen in profile, would seem to be very deeply sul- 
cated on each side of the keel, but this is due to the prominence of 
the row of nodes on either side of the same. Costa, when the shell 
has attained a diameter of four inches much depressed in the middle, 
with the nodes at their inner ends thicker and more obtuse, and those 
nearest the keel more depressed or nearly obsolete, while those of 
the third series, near by, become much enlarged and produced 
obliquely outward as short, thick, spine-like projections. Soon the 
outer compressed nodes disappear, and the keel is only represented 
by separated, low, elongated nodes; and when the shell has attained 
a diameter of seven inches, the coste are more distant, greatly ele- 
vated, compressed, and almost wing-like, but still retain a large, 
prominent, subtrigonal node or projection at their outer ends, and 
again become, as it were, pinched up at their inner extremities, 
which do not quite reach the umbilical margin. 
‘‘Septa moderately close together; siphonal lobe longerthan wide 
with three or four short branches on each side, the two terminal of 
which are largest, more or less nearly parallel, and merely serrated; 
first lateral sinus broader than the siphonal lobe, more or less deeply 
divided into two subequal branches with short, irregular branchlets 
and digitations; first lateral lobe somewhat longer than the siphonal, 
and tripartite, with short, irregular branchlets and digitations occa- 
sionally in small specimens, with the middle terminal branch propor- 
tionately broad and so deeply sinuous at the end as to impart more 
nearly the appearance of a bipartite arrangement of the whole; sec- 
