IOWA ACADEMY OF 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 geologic ^1 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 atteation is called includes a 

 huge Orthoceras — 0. fanslerensis —troui the lower coal meas- 

 ures' at Fansler, Guthrie county, Iowa. It may be briefly 

 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 alono 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, siz3 and abundance until at the close of the 

 Paleozoic the form was all but extinct. In the American Silu- 



