INVERTEBRATES. 161 



defined band, open umbilicus, and in not having the inner lip callous and spread 

 over the inner whorls. They seem also to have been thinner shells than the 

 typical Bellerophons, and may belong to an entirely distinct genus. 



Specifically, the form under consideration seems to approach B. Deslon- 

 champsii, d'Orbigny, from the Upper Silurian, (referred by that author to Cyrto- 

 lites, of Conrad), but has its dorsum more angular, and its aperture narrower, 

 than d'Orbigny's species. 



Locality and position: Rockford, Indiana; in the Goniatitc bed of the Kin- 

 derhook group. Subcarboniferous. 



CEPHALOPODA. 



Genus NAUTILUS, Linnceus, 1758. 



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



Subgenus TREMATODISCUS, M. and W. 



Trematodiscus, Meek and Worthen, 1861. Proceed. Acad. Nat. Sci., Philad'., p. 147. 



We proposed to range under the above subgeneric name, a peculiar group 

 of Nautili, which differ so materially from the living typical species of Nautilus, 

 that few Conchologists would place them even in the same genus, if they were 

 found inhabiting our present seas. These shells are characterized by a discoid 

 form, and a wide, shallow umbilicus, usually, if not always, perforated in the 

 middle, and showing all the inner turns. Their whorls are slender, very 

 slightly embracing or merely in contact, and provided with revolving angles 

 and grooves ; while the surface is frequently ornamented with distinct revolv- 

 ing strise. The siphuncle is central, or located between the middle and the 

 dorsal side, though never quite marginal. 



In first publishing descriptions of some of the forms included in this sub- 

 genus, we proposed to retain for it Prof. King's abandoned name, Discus. As 

 this name had, however, been previously used by Fitzinger in 1833, for a 

 group of Helicidee, and by Prof. Haldeman in 1840, for a group of Limnseidse, 

 it became necessary to introduce a new name for the forms under considera- 

 tion. 



Although in some respects analagous to Discitcs, of McCoy, these shells differ 

 from the typical forms of that group in having a perforated umbilicus ; while 

 the whorls are less compressed, narrower, and provided with several distinct 

 revolving angles and grooves. They also differ in being often ornamented with 

 well defined revolving lines. 



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