IOWA ACADEMY OP SCIENCES. 
77 
Shell attaining a large size, subdiscoidal; umbilicus large, or nearly- 
equaling the dorse- ventral diameter of the outer volution near the aper- 
ture; volutions three, enlarging their diameter more than three-fold each 
turn; all broader transversely than dorse-ventrally; inner ones slightly 
embracing, while the last one is apparently merely in contact with the 
others near the aperture; each broadly flattened or a little concave on the 
periphery, and (particularly the last one) somewhat flattened between the 
periphery and the middle of each side, from which point the sides are 
broadly rounded into the umbilicus, the greatest transverse diameter being 
near the middle; ventro-lateral or outer angles of the last whorl (in some- 
what worn casts), each provided with obscure traces of about twenty wide, 
undefined nodes, scarcely perceptible to the eye; septa numerous, rather 
closely arranged, making a slight backward curve on each side, particu- 
larly between the middle and outer angles and crossing the broadly flat- 
tened dorsum with a strong backward curve; surface with distinct lines of 
growth, which curve strongly backward like the septa, in crossing the 
outer side. 
White’s specimen was not as perfect; the recently acquired 
material, and consequently the latter, is of unusual interest as 
elucidating structural points which were previously obscure. 
The large dimensions which the shell attained is quite remark- 
able, especially when taken in comparison with the other forms 
of the group known from the same geologicil formation. Rarely 
do any of the species of the genus from the Carboniferous of 
the region reach a diametric measurement of more than four or 
five inches. The specimens of Nautilus ponderosus recently 
found are twelve to fifteen inches in diameter and weigh 
upwards of fifty pounds. 
The second group to which attention is called includes a 
huge Orthoceras — 0. fanslerensis— from the lower coal meas- 
ures at Fansler, Guthrie county, Iowa. It may be briefiy 
described as follows: 
Shell very large, thin, tapering very gradually; septa very thin, mod- 
erately concave, about two to the space of an inch near the large end; sur- 
face smooth. Diameter at larger extremity three inches, length probably 
not less than six feet. 
It is a well known fact that the straight- shelled cephalopod 
was an abundant form of life during Paleozoic times. This is 
attested by the large number of species that have been described, 
those of the Orthoceras group alone numbering over 1,200. The 
culmination and greatest expansion of the group was in the Silu- 
rian, and from that period it appears to have gradually dwindled 
in number of species, size and abundance until at the close of the 
Paleozoic the. form was all but extinct. In the American Silu- 
