386 PALEONTOLOGY OF ILLINOIS. 



CEPHALOPODA. 



Genus NAUTILUS, Linnaeus, 1758. 



(Syst. Nat. Ed., 10, t. 1, p. 709.) 



Nautilus planorbiformis, M. and W. 



PI. 29, fig. 4a, 4 6, 4c. 



Nautilus (Discus) planorbiformis, Meek and Worthen, Oct., 1860. Proceed. Acad. Nat. 

 Sci., Philad., p. 469. 



Shell of medium size, compressed-discoidal ; umbilicus very 

 wide, shallow, and showing nearly all of each inner whorl. 

 Volutions about four, increasing gradually in size, very slightly 

 embracing, nearly rounded in young shells, but becoming a 

 little compressed on the sides and dorsum in mature individu- 

 als; having a row of obscure nodes around each dorso-lateral 

 margin. Aperture slightly oval, its longer diameter being in 

 the plane of the shell. Septa deeply concave ; their margins 

 arching gently backwards on the sides and dorsum, and sepa- 

 rated by spaces less than one-third the transverse diameter of 

 the whorls ; siphuncle small and central ; surface apparently 

 smooth. Length or greatest diameter, 3.74 inches; height, 3.10 

 inches; breadth, about 1 inch. 



This and the following species, together with such forms as N. occidentalis 

 of Swallow, should probably constitute another subgenus distinct from Dis- 

 cites and Trematodiscus (see p. 161). We are rather inclined to doubt, how- 

 ever, the propriety of retaining either of these groups in the genus Nautilus, 

 as founded upon widely different recent shells. 



Locality and position : Alpine, Iowa; Coal Measures. 



Nautilus sangamonensis, M. and "W. 



• PI. 29, figs. 3, 3a, 3b. 



Nautilus (Discus) sangamonensis, Meek and Worthen, Oct., 1860. Proceed. Acad. Nat. 

 Sci., Philad., p. 470. 



Compare Discites tuberculatus, Owen, 1852. Geol. Report Wisconsin, Iowa and Min- 

 nesota, p. 581, tab. v, fig. 14; (not Nautilus tuberculatus, Sowerby, 1821). 



Of this species we have seen but a single specimen, consist- 

 ing of about half of one volution. It is a little wider trans- 



