64 PALAEONTOLOGY OF THE UPPER MISSOURI. 



Discites, Trematodiscus^ TemnocJieilus, 1 Northoceras, Pteronautilus, 2 Lituites? Hor- 

 tolus ? Cryptoceras, Clymenia, Sulclymenia, Aganides (= Aturia), Nautiloceras, 

 Aploceras, and Trochoceras, with probably some undescribed Jurassic, Triassic and 

 older genera. 



GENUS NAUTILUS, LINN^US. 



Synon. Nautilus, BBEIHIOS, Dissert. Polyth. 1732, 12-14. LIHN. (part), Syst. Nat. ed. 10, 1758, t. i, 709. BBUG. 

 Encyc. Meth. I, 1789, p. xvi. LAMK. Prodr. 1799, 79 ; and Syst. 1801, 99. 



Oceanus, MONTF. Conch. Syst. 1808, p. 58-9. 



Ammonites, IB. 74-5, not (BKEYN. LINN.KOS). 



Ellipsolithes (sp.), SOWEBBY, Min. Conch. I, 1814, 56 (not MOXTF. 1808). 



Omphalia and Nautilus, DE HANN, Mon. Amm. 1824. 



Simplegas (sp.), BLAINVILLE, Diet. Soi. Nat. torn. 32, 185 (not Simplegades, MOJTTF. 1808). 

 Etym. MimX*, a sailor or navigator. 

 Type. N. pomiiilius, LINN.'^INN. 



Shell subglobose or more or less compressed ; umbilicus closed or open ; volu- 

 tions coiled in the same plane, merely contiguous, or more or less deeply embracing. 

 Septa simple or somewhat arched or waved on the lateral margins; siphon 

 generally central or subcentral. Surface smooth or variously striate or costate 

 and furrowed ; in some of the older extinct species ornamented with nodes. Lip 

 generally more or less sinuous on the dorsal and ventro-lateral margins. 



1 In the June number of the Proceed. Acad. Nat. Sci. Phila. 1862, p. 147, the subgenerie name 

 Trematodiscus was proposed by the writer and Mr. Worthen, for such fossil species as Nautilus 

 strigalis, N. Edwardsianus and N. omalianus, Koninck ; and N. sulcatus, N. pinguis, &c., Sowerby. 

 Since that time we observe Prof. McCoy had proposed, in 1844 (Garb. Foss. Ireland), the name 

 Temnocheilus for a group including some of the same forms. .As his first species, however (N. 

 biangulatus, Sowerby), and several of the others ranged by him under that name, differ considerably 

 in form and surface characters from the types for which the name Trematodiscus was proposed, it 

 may be convenient to restrict Temnocheilus to such forms as N. biangulalus, Sowerby ; and N. 

 pinguis, and N. coronatus, McCoy ; and to retain Trematodiscus for those agreeing with its type as 

 originally proposed. If it should be thought desirable, however, to group all these forms together, 

 Prof. McCoy's name Temnocheilus would have to take precedence, as it was first published. 



It is also worthy of note that Blainville had, in 1824 (Diet. Sci. Nat. torn. 32, p. 185) included 

 some similar forms under the name Simplegas, adopted from Montfort. It will be remembered, 

 however, that the type of Montfort's genus Simplegades (not Simplegas), is an Ammonite or Cera- 

 tite ; while the first, or typical species of Simplegas, Blainville, is a true Nautilus. 



In 1842, Mr. G. B. Sowerby adopted, in his Manual (p. 276), Blainville's name Simplegas, and 

 figures as an illustration of the group N. sulcatus, Sowerby, a typical Trematodiscus. Although he 

 writes the name Simplegas, he cites Montfort as the author of the group, and yet admits that the 

 type of Simplegades, Montfort, has sinuous septa like Ammonites. 



* The name Pteronautilus is proposed for a remarkable undescribed Permian genus, of which 

 Nautilus Seebachianus, Genitz, is the type (see Dyas, p. 43, tab. 11). It may be characterized as 

 follows: 



GENPS Pteronantilus, MEEK. 



Shell with the involute body portion comparatively very small and globular in form, scarcely umbilicate. Outer 

 chamber very large, and deflected from the involute body, its inner or ventral side being widely open, and the 

 lateral margins greatly dilated, so as to form a very large wing-like expansion on each side. 



Conchologists will readily understand that such a shell as this must have been inhabited by an 

 animal differing widely in its structure from a living typical Nautili. 



